feat: add worktree isolation to /implement, /validate, and /release

[frosty-geek]

Summary

• /implement now creates a dedicated git worktree at .claude/worktrees/<epic-slug> before dispatching the taskplanner, enabling parallel feature implementation
• /validate now enters the matching worktree before running tests, so it validates the correct code even when invoked from a fresh session
• /release now enters the matching worktree before generating release notes and dispatching the retro context walker, so the retro reads/writes the feature branch's state and its edits land in the squash-merge
• Moved the TRANSITION BOUNDARY callout in /release Phase 3 from above Step 2 to above Step 3 — Step 2 (Commit to Feature Branch) still runs in the worktree; the actual transition to the main repo is at Step 3 (Squash Merge to Main)

Closes #564

Changes

plugin/skills/implement/SKILL.md

• Updated working directory isolation principle to reflect skill-managed worktrees
• Added Phase 0: Worktree Setup with existing-worktree check (0a) and create-worktree (0b) steps

plugin/skills/validate/SKILL.md

• Added Step 0: Enter Worktree to Phase 0 (Pre-Execute)
• Checks git worktree list, enters matching worktree, or blocks if none exists

plugin/skills/release/SKILL.md

• Added Phase 0: Pre-Execute with Step 0: Enter Worktree, mirroring /validate's pattern
• Without this, /release invoked from a fresh session at the repo root runs Phase 1 and Phase 3 Step 1 (retro context walker) from the main repo, not the worktree. The retro is dispatched as a general-purpose subagent that inherits the parent's cwd; with no Phase 0 entry, it reads main's .beastmode/artifacts/* and writes to main's .beastmode/context/* — the worktree's edits never make it through the squash-merge
• Moved the TRANSITION BOUNDARY callout from above Phase 3 Step 2 to above Step 3 (Step 2 is the feature-branch checkpoint commit, which still runs in the worktree; only Step 3+ navigates to the main repo via explicit cd)

Context

Without these changes, running /implement on two features simultaneously corrupts the working tree, running /validate in a new session tests main instead of the feature branch, and the /release retro context walker silently writes to whichever directory the session happens to be in (often main, where the edits get clobbered by the subsequent squash-merge from the worktree). All three were discovered while running parallel beastmode sessions on a real project.

Test plan

• Run /implement on a feature — verify worktree is created at .claude/worktrees/<epic-slug>
• Run /implement on a second feature in parallel — verify each gets its own worktree
• Run /validate in a fresh session — verify it finds and enters the worktree
• Run /validate with no worktree — verify it blocks with a clear message
• Run /release in a fresh session — verify it enters the worktree before the retro fires
• Run /release with no worktree — verify it blocks with a clear message
• Inspect the squash-merge commit — verify retro context edits (e.g., updates to .beastmode/context/*.md) are present on main