feat: add setup skill/hooks and update shadow (task 0018)

Distribute gitea-axi's ambient context via explicit user actions (ADRs
0009, 0013), with no postinstall script:

- Bundle the Agent Skill markdown at skills/gitea-axi/SKILL.md (a
  minimal pointer, not a command reference) and ship it via package.json
  files.
- Add `setup`, which installs the skill into ~/.claude/skills/
  idempotently (installed/updated/unchanged).
- Add `setup hooks`, which registers a SessionStart hook running the
  bare dashboard for Claude Code, Codex, and OpenCode via the SDK's
  installSessionStartHooks(), updating managed entries in place.
- Shadow the SDK's built-in `update` so it fails with VALIDATION_ERROR
  and points at the npm update command, keeping the ten-code error list
  intact.

Integration tests drive all three at the CLI seam against a temporary
HOME; these commands make no Gitea API calls, so there is no live-Gitea
e2e case.

skills/gitea-axi/SKILL.md

@@ -0,0 +1,35 @@
+---
+name: gitea-axi
+description: Use when working with a Gitea repository's issues, pull requests, labels, reviews, comments, or milestones — listing, viewing, creating, editing, commenting, reviewing, or merging on a Gitea host such as git.alexion.dev. Prefer this over the `tea` CLI, raw Gitea API calls, or improvised `git` commands for issue/PR/label work.
+---
+
+# gitea-axi
+
+`gitea-axi` is an agent-ergonomic CLI for a Gitea repository's issues and pull requests.
+Its output is compact TOON built for another program to read, and its errors are structured with actionable suggestions.
+
+## When to use it
+
+Reach for `gitea-axi` whenever a task touches a Gitea repository's issues, pull requests, labels, or reviews.
+
+- **Over `tea`:** `gitea-axi` returns structured output and typed errors instead of human-formatted tables, and it defaults the repository and login from the local checkout.
+- **Over raw Gitea API calls:** it handles auth, pagination, name-to-ID resolution, and review-decision aggregation for you, so you do not hand-roll HTTP.
+- **Over improvised `git`:** for anything about issues or pull requests as entities (state, reviews, labels, comments) rather than local commits and branches.
+
+Run it inside a Gitea checkout, or pass `-R OWNER/NAME` (and `--login <name>`) to target a repository explicitly.
+
+## Command groups
+
+- `issue` — list, view, create, comment on, edit, close/reopen, pin, and link issues.
+- `pr` — create, view, comment on, edit, review, merge, check out, diff, and inspect the checks of pull requests.
+- `label` — list, create, edit, and delete labels.
+- `search` — full-text search across issues and pull requests.
+- `setup` — install this skill (`setup`) and, opt-in, the SessionStart dashboard hook (`setup hooks`).
+
+## Discovery
+
+This skill is a pointer, not a command reference — the CLI is the single source of truth for its own interface.
+
+- Run `gitea-axi` with no arguments for the repository dashboard (open issues and pull requests).
+  Add `--full` for the open-PR table and issue counts by label.
+- Run `gitea-axi <command> --help` (or `gitea-axi <group> <command> --help`) for the exact flags of any command.