feat: add nimble remove command to uninstall community skills

[adeptofvoltron]

Summary

• Adds nimble remove <skill-name> CLI command — the missing counterpart to nimble add
• Removes the skill entry from config.yaml, its lock entry from manifest.lock, and deletes the .nimble/skills/<name>/ directory in a single step
• Requires explicit y/N confirmation before any state is mutated; gracefully warns (no abort) if the directory or lock entry is already absent; prints a nimble restart hint when the daemon is running

Test plan

• pytest tests/unit/manifest/test_lock.py tests/unit/cli/test_commands.py -q — 63 tests, all green
• flake8 nimble/manifest/lock.py nimble/manifest/parser.py nimble/cli/commands.py — clean
• mypy nimble/manifest/lock.py nimble/manifest/parser.py nimble/cli/commands.py — clean
• Manual: nimble add a skill, then nimble remove <name> — confirm config entry, lock entry, and directory are gone
• Manual: nimble remove nonexistent — confirm exits 1 with "not found" message
• Manual: nimble remove <name> with daemon running — confirm restart hint appears

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