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>

.skillignore

@@ -0,0 +1,33 @@
+# Agent Skills install-time scanner exclusions for repository-root scans.
+# Keep the public bundle focused on the runtime skill under skills/watch/.
+
+# VCS, local envs, caches, and generated outputs
+.git/
+.venv/
+__pycache__/
+*.pyc
+*.log
+.DS_Store
+.pytest_cache/
+.coverage
+htmlcov/
+dist/
+
+# Repo/dev automation and host-specific package metadata
+.github/
+.agents/
+.claude-plugin/
+hooks/
+dev-sync.sh
+
+# Non-runtime docs, plans, and tests
+tests/
+README.md
+CHANGELOG.md
+AGENTS.md
+CLAUDE.md
+V2_PLAN.md
+V2_CONCERNS.md
+
+# Dev-only script shipped inside the skill tree but not needed at runtime
+skills/watch/scripts/build-skill.sh