docs: document interrupted-worktree recovery and gitignore dirty-detection

[baruch]

What

Adds user- and agent-facing docs for two behaviors that were previously only
derivable by reading the code.

1. Recovering an interrupted worktree (README)

1. Recovering an interrupted worktree (README)

New "Recovering an interrupted worktree" section covering what happens when
a treehouse get subshell ends abnormally — closed terminal, crash, or OOM:

• The normal return cleanup is skipped, the stale owner reservation is
  self-healed on the next command, and uncommitted work (tracked or
  untracked) stays protected from get reuse, prune, and destroy.
• How to recover: treehouse status to find the dirty worktree by path,
  cd in to resume, treehouse return <path> when done.
• A caveat flagging the two clean-tree cases that are not protected on a
  non-leased worktree — committed work (reclaimed by git reset --hard,
  recoverable via git reflog) and .gitignored-only changes — and pointing
  to treehouse get --lease for durable holds across crashes.

2. .gitignored files vs. dirty detection (README + AGENTS.md)

Clarifies a previously-undocumented edge case: ignored files are not counted
as dirty (so a worktree whose only leftover content is ignored can be reused),
but treehouse's reset never deletes them either — ResetWorktree runs
git clean -fd (without -x), which respects .gitignore.

Why

The dirty-detection note previously said only "untracked files count as dirty."
The crash-recovery flow and the ignored-files nuance are accurate in the code
(IsDirty, ResetWorktree, healState, Acquire) but were undocumented —
exactly the behavior a user relies on when an agent session dies.

Scope

Documentation only — no code changes. Touches README.md and AGENTS.md
(the design-decisions note; CLAUDE.md symlinks to it). 2 files, +26/−2.

[kunchenguid]

Thanks for the PR! This branch currently has a merge conflict with the base branch.

When you get a chance, please rebase onto (or merge) the latest base branch, resolve the conflict, and push. After that, checks will re-run and the PR will get looked at again.

_{Noted for treehouse#40 at d0e6b1b3.}

[kunchenguid]

Automated reminder: this PR still looks blocked on a rebase or merge conflict fix.

If you are still interested, please rebase onto the current base branch, resolve the conflict, and push.

If I do not hear back, I may close this as inactive.

[kunchenguid]

I am closing this because it has been waiting on a rebase or merge-conflict fix since 2026-07-10, and I have not seen a comment or push since then.

If you still want to keep working on this, please reopen it or open a new PR and mention this one.

Happy to take another look when there is an update.