Add herdr plugin integration and WT_* hook environment

[bkildow]

Summary

• wt herdr init installs a global herdr plugin. Its worktree.created hook applies shared files and runs setup hooks in herdr-created worktrees; its worktree.removed hook handles cleanup. Project root is resolved from the event's repo_key, so it works for both .bare and .git layouts and for checkouts outside the project tree.
• post_remove hooks (new .worktree.yml key). herdr deletes the checkout before emitting worktree.removed, so ordinary teardown hooks are skipped there with a warning rather than run against the wrong directory. post_remove runs from the project root with WT_* exported, e.g. ddev delete -Oy "$(basename "$WT_WORKTREE_PATH")".
• Setup/removal race: the created hook records its PID in herdr's plugin state dir; the removed hook stops and waits on a still-running setup before cleanup.
• WT_* environment for all hooks: WT_PROJECT_ROOT, WT_WORKTREE_ID, WT_WORKTREE_PATH, WT_BRANCH_NAME are now exported to every hook across wt add, wt setup, wt remove, wt prune, the Claude Code hooks, and the herdr hooks. Hook runners take TemplateVars instead of a bare path.
• Docs: README sections for wt herdr init, hook environment variables, and post-remove hooks; integrations/herdr/README.md records what herdr 0.9.0 actually does, with captured payloads.

Test plan

• make dev (fmt, lint, tests, build) clean
• Unit tests: event parsing, project-root resolution for both layouts, quiet no-op on non-wt repos, teardown skipped when dir gone / run when present, in-flight setup stopped, WT_* env visible to setup/teardown/post_remove
• Live against herdr 0.9.0: sidebar-equivalent create → shared symlinks + setup in checkout; remove → post_remove from project root; remove during a 20s setup → setup killed, no stray process, pid file cleaned
• Live wt add (foreground and background) and wt remove show correct per-worktree WT_* values
• ddev delete <name> confirmed to work after the project directory is deleted (basis for the post_remove example)

https://claude.ai/code/session_01DYtq4h1YVJrotQVtc4bpDR