feat(subject): add pr/issue set-subject via NIP-32 kind-1985 labels

Adds the ability to update the displayed title of a PR or issue after
creation using a kind-1985 label event with the #subject namespace.
Only the author or a repository maintainer may set the subject. The
latest authorised event wins with tiebreak by lexicographically larger
event ID (NIP-1 replaceable event semantics). Branch names and commit
messages are never affected.

- Split get_labels() into process_labels() (additive #t) and
  process_subject() (replaceable-style #subject), with a shared
  get_labels_and_subject() entry point that processes both from a
  single pre-fetched slice of kind-1985 events
- All list/view/JSON display paths apply the subject override silently
- New ngit pr set-subject <id> --subject <text> command
- New ngit issue set-subject <id> --subject <text> command

1 files changed, 120 insertions, 13 deletions

diff --git a/src/lib/git_events.rs b/src/lib/git_events.rs
index a5aef12..b512e44 100644
--- a/src/lib/git_events.rs
+++ b/src/lib/git_events.rs
@@ -978,30 +978,31 @@ pub fn is_event_proposal_root_for_branch(
     ))
 }
 
-/// Compute the effective set of labels for `event`.
+/// Process hashtag labels (`#t` namespace) from a pre-fetched set of kind-1985
+/// events.
 ///
 /// Labels come from two sources, both subject to the same permission check:
 ///
 /// 1. `t` tags on the event itself (self-reported by the event author).
-/// 2. NIP-32 kind-1985 label events in `all_label_events` that reference
-///    `event` via a lowercase `e` tag and carry `["L", "#t"]` +
+/// 2. NIP-32 kind-1985 label events in `label_events` that reference `event`
+///    via a lowercase `e` tag and carry `["L", "#t"]` +
 ///    `["l", "<value>", "#t"]` tags.
 ///
-/// A label is only applied when the author of the source event (the original
-/// event for inline `t` tags, or the kind-1985 event for external labels) is
-/// either the author of `event` itself or one of the repository maintainers.
-pub fn get_labels(
+/// A label is only applied when the author of the source event is either the
+/// author of `event` itself or one of the repository maintainers.
+///
+/// Labels are additive — all valid label events contribute; there is no
+/// "latest wins" replacement semantics.
+pub fn process_labels(
     event: &Event,
     repo_ref: &RepoRef,
-    all_label_events: &[Event],
+    label_events: &[Event],
 ) -> Vec<String> {
     let is_permitted = |pubkey: &PublicKey| -> bool {
         pubkey.eq(&event.pubkey) || repo_ref.maintainers.contains(pubkey)
     };
 
-    // 1. Inline `t` tags on the event itself — only if the event author is
-    //    permitted (they always are, since they authored the event, but we
-    //    keep the check symmetric with the external-label path).
+    // 1. Inline `t` tags on the event itself.
     let mut labels: Vec<String> = if is_permitted(&event.pubkey) {
         event
             .tags
@@ -1016,7 +1017,7 @@ pub fn get_labels(
         vec![]
     };
 
-    // 2. External NIP-32 kind-1985 label events.
+    // 2. External NIP-32 kind-1985 label events (`#t` namespace).
     //
     // A valid label event must:
     //   - be kind 1985
@@ -1025,7 +1026,7 @@ pub fn get_labels(
     //   - have at least one `["l", "<value>", "#t"]` tag
     //   - be authored by a permitted pubkey
     let event_id_str = event.id.to_string();
-    for label_event in all_label_events {
+    for label_event in label_events {
         if !label_event.kind.eq(&KIND_LABEL) {
             continue;
         }
@@ -1063,6 +1064,112 @@ pub fn get_labels(
     labels
 }
 
+/// Process the effective subject/title override for `event` from a pre-fetched
+/// set of kind-1985 events.
+///
+/// Subject overrides use the `#subject` namespace:
+///   `["L", "#subject"]` + `["l", "<new title>", "#subject"]`
+///
+/// Unlike hashtag labels, subject overrides are replaceable-style: only the
+/// latest authorised event wins, with tiebreak by lexicographically larger
+/// event ID (consistent with NIP-1 replaceable event semantics).
+///
+/// Only the author of `event` or a repository maintainer may set the subject.
+/// Returns `None` when no valid subject override exists.
+pub fn process_subject(
+    event: &Event,
+    repo_ref: &RepoRef,
+    label_events: &[Event],
+) -> Option<String> {
+    let is_permitted = |pubkey: &PublicKey| -> bool {
+        pubkey.eq(&event.pubkey) || repo_ref.maintainers.contains(pubkey)
+    };
+
+    let event_id_str = event.id.to_string();
+
+    // Find the winning subject label event: latest created_at, tiebreak by
+    // lexicographically larger event ID (NIP-1 replaceable event semantics).
+    let winner = label_events
+        .iter()
+        .filter(|le| {
+            if !le.kind.eq(&KIND_LABEL) {
+                return false;
+            }
+            if !is_permitted(&le.pubkey) {
+                return false;
+            }
+            // Must reference our event via a lowercase `e` tag.
+            let references_event = le.tags.iter().any(|t| {
+                let s = t.as_slice();
+                s.len() >= 2 && s[0].eq("e") && s[1].eq(&event_id_str)
+            });
+            if !references_event {
+                return false;
+            }
+            // Must declare the `#subject` namespace.
+            let has_namespace = le.tags.iter().any(|t| {
+                let s = t.as_slice();
+                s.len() >= 2 && s[0].eq("L") && s[1].eq("#subject")
+            });
+            if !has_namespace {
+                return false;
+            }
+            // Must have at least one non-empty `["l", "<value>", "#subject"]` tag.
+            le.tags.iter().any(|t| {
+                let s = t.as_slice();
+                s.len() >= 3 && s[0].eq("l") && s[2].eq("#subject") && !s[1].is_empty()
+            })
+        })
+        .max_by(|a, b| {
+            // Primary: newer created_at wins.
+            // Tiebreak: lexicographically larger event ID wins (NIP-1).
+            a.created_at
+                .cmp(&b.created_at)
+                .then_with(|| a.id.to_string().cmp(&b.id.to_string()))
+        })?;
+
+    // Extract the subject value from the winning event.
+    winner.tags.iter().find_map(|t| {
+        let s = t.as_slice();
+        if s.len() >= 3 && s[0].eq("l") && s[2].eq("#subject") && !s[1].is_empty() {
+            Some(s[1].clone())
+        } else {
+            None
+        }
+    })
+}
+
+/// Compute both the effective hashtag labels and the subject/title override for
+/// `event` from a pre-fetched set of kind-1985 events.
+///
+/// This is the primary entry point: callers should fetch label events once
+/// (covering both `#t` and `#subject` namespaces) and pass them here to get
+/// both results in a single pass.
+///
+/// Returns `(labels, subject_override)` where `subject_override` is `None`
+/// when no authorised `#subject` label exists.
+pub fn get_labels_and_subject(
+    event: &Event,
+    repo_ref: &RepoRef,
+    label_events: &[Event],
+) -> (Vec<String>, Option<String>) {
+    (
+        process_labels(event, repo_ref, label_events),
+        process_subject(event, repo_ref, label_events),
+    )
+}
+
+/// Compatibility wrapper — returns only the hashtag labels.
+///
+/// Prefer [`get_labels_and_subject`] when the subject override is also needed.
+pub fn get_labels(
+    event: &Event,
+    repo_ref: &RepoRef,
+    label_events: &[Event],
+) -> Vec<String> {
+    process_labels(event, repo_ref, label_events)
+}
+
 pub fn get_status(
     proposal: &Event,
     repo_ref: &RepoRef,