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.

test/update.test.ts

@@ -0,0 +1,13 @@
+import { describe, expect, it } from "vitest";
+import { runCliTest } from "./harness.js";
+
+describe("update (shadowed self-update)", () => {
+  it("refuses to self-update: VALIDATION_ERROR, exit 2, npm help line, no SDK UPDATE_ERROR", async () => {
+    const { stdout, exitCode } = await runCliTest(["update"]);
+
+    expect(exitCode).toBe(2);
+    expect(stdout).toContain("code: VALIDATION_ERROR");
+    expect(stdout).toContain("npm install -g gitea-axi@latest");
+    expect(stdout).not.toContain("UPDATE_ERROR");
+  });
+});