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, 2 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b2d5617..a209922 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - NIP-32 label support: kind-1985 label events are now fetched alongside status events and merged with inline `t` tags to compute the effective label set for issues and PRs; only labels authored by the issue/PR author or a repository maintainer are applied; label counts appear in the fetch progress report
 - `ngit issue label <id> --label <L> [--label <L>...]` — apply one or more NIP-32 hashtag labels to an existing issue (author or maintainer only); publishes a kind-1985 event; duplicate labels already present via `t` tags or prior kind-1985 events are silently skipped; the new event is saved to the local cache before broadcasting so subsequent reads reflect the change immediately
 - `ngit pr label <id> --label <L> [--label <L>...]` — apply one or more NIP-32 hashtag labels to an existing PR (author or maintainer only); publishes a kind-1985 event; duplicate labels already present via `t` tags or prior kind-1985 events are silently skipped; the new event is saved to the local cache before broadcasting so subsequent reads reflect the change immediately
+- `ngit pr set-subject <id> --subject <text>` — update the displayed title of a PR via a NIP-32 kind-1985 `#subject` label event (author or maintainer only); the latest authorised subject takes precedence with tiebreak by event ID; does not affect the underlying git commit messages or branch names
+- `ngit issue set-subject <id> --subject <text>` — update the displayed title of an issue (author or maintainer only); same semantics as PR set-subject
 - `ngit account whoami` — show the currently logged-in account(s)
 - `ngit pr` subcommand group: `list`, `view`, `checkout`, `apply`, `send`, `close`, `reopen`, `ready`, `draft`, `comment`, `merge`; replaces the former top-level `ngit list`, `ngit checkout`, and `ngit apply` commands (hard-migrated); `ngit send` remains at the top level unchanged
 - `ngit pr view <id>` — view a PR with its full details and all comments (author, timestamp, body) in chronological order