Add a gitea skill; point AGENTS.md at it

Forge usage was spread across AGENTS.md, docs/agents/issue-tracker.md, and
whatever the agent remembered about `gh`. Move the how-to-run-`tea` half into a
model-invoked skill so it loads on any issue/PR task, and leave AGENTS.md a
single line of identity plus pointers.

The skill carries only what `tea --help` cannot answer: `gh` is absent so the
harness's `issue://`/`pr://` URIs error out, issues and PRs share one index
space, rendered-box output needs `-o json`, `close` takes no `--comment`,
labels never auto-create, bodies need a heredoc, and a read without
`--comments` silently drops every comment (issue #123: 40 lines bare, 132 with
the flag). Tracker conventions and triage label strings stay in
docs/agents/, cited rather than copied.

.claude/skills/implement-tickets/SKILL.md

@@ -11,7 +11,7 @@ to the tracker. You do not write the implementation — every line of ticket cod
 is written by a `ticket-implementer` subagent in its own git worktree. Reach for
 the editor yourself only for a merge conflict resolution.
 
-Ticket source and `tea` usage: `docs/agents/issue-tracker.md`.
+Ticket source and tracker conventions: `docs/agents/issue-tracker.md`. `tea` usage: skill `gitea`.
 
 ## 1. Collect the tickets