Restructure as a self-contained Agent Skills package (fix Codex install)

Move the skill into skills/watch/ so SKILL.md and its scripts/ runtime are
siblings inside one folder. `npx skills add` (Codex/Cursor/Copilot/agents) now
copies a working skill as a unit; previously it grabbed the root SKILL.md alone
and left scripts/ behind, so the skill was dead on arrival on every non-Claude
host. Mirrors the layout last30days-skill adopted for the same reason.

- skills/watch/{SKILL.md,scripts/}: self-contained skill folder
- SKILL.md: resolve a harness-agnostic $SKILL_DIR (the dir it was Read from)
  instead of the Claude-Code-only ${CLAUDE_SKILL_DIR}; guard + 19 call sites
- drop commands/watch.md: /watch derives from frontmatter (name + user-invocable)
- .codex-plugin/plugin.json: full manifest with "skills": "./skills/" + interface
- add .agents/plugins/marketplace.json, AGENTS.md, CLAUDE.md, .skillignore
- build-skill.sh: archive the skills/watch subtree (one SKILL.md, no zip -d)
- fix paths in tests, hooks hint, .gitattributes, release.yml
- relocate dev-sync.sh to repo root and fix REPO_ROOT
- README: content-ideas structure, npx skills install, star history

Verified: 37/37 tests pass; npx skills add bundles the full scripts/ runtime;
manifests valid; versions synced at 0.1.3.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitattributes

@@ -1,5 +1,6 @@
 # Exclude non-runtime files from `git archive` output.
-# Used by scripts/build-skill.sh to produce a claude.ai-upload-ready .skill file.
+# Used by skills/watch/scripts/build-skill.sh to produce the claude.ai .skill file
+# and by Claude Code's /plugin install (full-repo archive).
 
 # Anthropic canonical skill-packaging excludes
 # (mirrors anthropics/skills/skills/skill-creator/scripts/package_skill.py)
@@ -15,10 +16,21 @@ fixtures/      export-ignore
 assets/        export-ignore
 examples/      export-ignore
 
-# NOTE: commands/, hooks/, and .claude-plugin/ are NOT export-ignored because
-# Claude Code's /plugin install fetches this same git archive tarball — they
-# must be in the archive for the plugin to install correctly. The claude.ai
-# .skill bundle strips them afterward via `zip -d` in scripts/build-skill.sh.
+# Dev tooling and planning notes — repo-only, not shipped to any install surface
+dev-sync.sh      export-ignore
+V2_PLAN.md       export-ignore
+V2_CONCERNS.md   export-ignore
+.agents/         export-ignore
+
+# Dev-only build script + scanner config inside the skill tree: keep them out of
+# the claude.ai .skill bundle (built via `git archive HEAD:skills/watch`).
+skills/watch/scripts/build-skill.sh export-ignore
+skills/watch/.skillignore           export-ignore
+
+# NOTE: hooks/ and .claude-plugin/ are NOT export-ignored because Claude Code's
+# /plugin install fetches the full-repo git archive — they must be present for
+# the plugin to install correctly. The claude.ai .skill bundle is built from the
+# skills/watch/ subtree only, so it never sees them in the first place.
 
 # CI workflows — repo-only, not needed at skill runtime
 .github/         export-ignore