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/skills/ngit/SKILL.md b/skills/ngit/SKILL.md
index c316c1e..272f9ba 100644
--- a/skills/ngit/SKILL.md
+++ b/skills/ngit/SKILL.md
@@ -124,6 +124,7 @@ ngit pr reopen <ID|nevent> --reason "fix was incomplete"
 ngit pr ready <ID|nevent> --reason "addressed review feedback"
 ngit pr draft <ID|nevent> --reason "needs more work"
 ngit pr label <ID|nevent> --label bug --label enhancement
+ngit pr set-subject <ID|nevent> --subject "New title"
 ```
 
 ## Issues
@@ -142,6 +143,7 @@ ngit issue close <ID|nevent> --reason "wontfix"
 ngit issue resolved <ID|nevent> --reason "fixed in abc123"
 ngit issue reopen <ID|nevent> --reason "regression in v2.3"
 ngit issue label <ID|nevent> --label bug --label enhancement
+ngit issue set-subject <ID|nevent> --subject "New title"
 ```
 
 ## Account management