feat(phase-5): Claude Code skill marketplace install path

[sonpiaz]

Summary

Adds `.claude-plugin/marketplace.json` so Claude Code users can install watch-cli as a skill via:

```
/plugin marketplace add sonpiaz/watch-cli
/plugin install watch-cli@watch-cli
```

The marketplace.json points at the existing `skills/watch-cli/SKILL.md` (symlinked from the repo-root canonical SKILL.md authored in Phase 2). No new skill content — this is purely the plumbing that makes the existing portable SKILL.md installable through Claude Code's plugin manager without manual `cp -r ~/.claude/skills/`.

Files changed

File	Change
`.claude-plugin/marketplace.json` (new)	Marketplace manifest pointing at `./skills/watch-cli`
`README.md` Install section	Adds Claude Code subsection; removes the now-obsolete "requires v0.3.0 to be cut first" caveat since v0.3.0 has shipped

Why this matters

Phase 2 shipped a portable `SKILL.md` valid in Claude Code + OpenClaw + Hermes. Until this PR, Claude Code users had to:

```
git clone https://github.com/sonpiaz/watch-cli ~/.claude/skills/watch-cli
```

After this PR, two slash commands handle it.

Test plan

• Render README on GitHub, confirm new Claude Code subsection
• Verify `/plugin marketplace add sonpiaz/watch-cli` resolves the manifest (will work once PR is merged + on `main`)
• No regressions in existing brew/curl install paths