docs: clarify safe worktree stacking

[nedtwigg]

Stacking with git switch -c reuses a worktree that another session may still need. Document creating a separate worktree with wt switch --create --base @, preserving active worktrees, and using an alternate path when the expected path is occupied.

Also clarify explicit working directories for agent commands and pnpm install for fresh worktrees; adjust the AGENTS.md word budget.

Validation: node scripts/spec-lint.mjs and git diff --check pass.

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[dormouse-bot]

The merge you just pushed resolves the budget correctly — 3350 covers the merged AGENTS.md at 3323 words, and node scripts/spec-lint.mjs is green on this head. One thing left.

wt isn't named anywhere else in the repo. Grepping the tree for it returns only these two lines — no install step, no link, nothing in Setup. AGENTS.md is the first thing an agent reads, so a Must rule whose verb is a command the reader doesn't have and can't look up is unactionable for exactly the audience the section is written for. Suggestion inline.

The second bullet also has a question in it: --config-set reads as writing persistent configuration, so it isn't clear whether that command permanently repoints worktree-path for every later wt switch in this repo or only for that one invocation — and .REAL carries no explanation, leaving a reader unsure whether it's a literal suffix or a placeholder to substitute. If it's a per-invocation override, saying so is the difference between a safe escape hatch and a change that silently relocates future worktrees.

[dormouse-bot]

wt needs an introduction here — it is worktrunk, and nothing else in the repo says so.

Suggested change

	- **Must stack with `wt switch --create <branch> --base @`**, never `git switch -c` in an existing worktree; other sessions need its branch and path intact. Set the GitHub PR base separately.
	`wt` is [worktrunk](https://github.com/max-sixty/worktrunk), installed separately.
	- **Must stack with `wt switch --create <branch> --base @`**, never `git switch -c` in an existing worktree; other sessions need its branch and path intact. Set the GitHub PR base separately.