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, 26 insertions, 0 deletions

diff --git a/src/bin/ngit/cli.rs b/src/bin/ngit/cli.rs
index a240597..8cdbee1 100644
--- a/src/bin/ngit/cli.rs
+++ b/src/bin/ngit/cli.rs
@@ -334,6 +334,19 @@ pub enum PrCommands {
         #[arg(long)]
         offline: bool,
     },
+    /// set the subject/title of a PR (author or maintainer only)
+    #[command(name = "set-subject")]
+    SetSubject {
+        /// Proposal event-id (hex) or nevent (bech32)
+        #[arg(value_name = "ID|nevent")]
+        id: String,
+        /// New subject/title for the PR
+        #[arg(long, alias = "title")]
+        subject: String,
+        /// Use local cache only, skip network fetch
+        #[arg(long)]
+        offline: bool,
+    },
 }
 
 // ---------------------------------------------------------------------------
@@ -461,6 +474,19 @@ pub enum IssueCommands {
         #[arg(long)]
         offline: bool,
     },
+    /// set the subject/title of an issue (author or maintainer only)
+    #[command(name = "set-subject")]
+    SetSubject {
+        /// Issue event-id (hex) or nevent (bech32)
+        #[arg(value_name = "ID|nevent")]
+        id: String,
+        /// New subject/title for the issue
+        #[arg(long, alias = "title")]
+        subject: String,
+        /// Use local cache only, skip network fetch
+        #[arg(long)]
+        offline: bool,
+    },
 }
 
 #[derive(Subcommand)]