Replace LEGACY_SKILL_NAMES with manifest-based stale skill sweep

[dean0x]

Problem

LEGACY_SKILL_NAMES in src/cli/plugins.ts is a 197-entry append-only array tracking every skill rename/removal since v1. It grows with each refactor and will only get larger as rules accumulate their own LEGACY_RULE_NAMES.

Proposed Solution

Replace the static array with a manifest-based sweep during devflow init (or uninstall):

1. Read the current manifest's installed skill list
2. Scan ~/.claude/skills/devflow:*/ for all installed skill directories
3. Remove any directory not in the current manifest's skill set
4. Same pattern for ~/.claude/rules/devflow/*.md and ~/.claude/agents/devflow/*.md

This eliminates the need for LEGACY_SKILL_NAMES, LEGACY_RULE_NAMES, and any future legacy arrays entirely — the manifest is the source of truth for what should exist.

Considerations

• Must handle partial installs (single-plugin reinstall shouldn't wipe skills from other plugins)
• Should log what it removes for transparency
• Could run as a migration or as part of normal init flow
• The current array approach is harmless (~4KB, runs once during uninstall) — this is a cleanliness improvement, not urgent

Scope

• Remove LEGACY_SKILL_NAMES array
• Remove LEGACY_RULE_NAMES array
• Add manifest-based sweep to init/uninstall flow
• Add tests for the sweep logic