chore(scripts): add setup-worktree.sh with copy-on-write node_modules

[kkohtaka]

Summary

• Worktrees under .claude/worktrees/ were filling the disk. The checkout itself is only ~3MB (git objects are shared with the main repo), but npm ci in each worktree costs ~424MB — a handful of accumulated worktrees had reached 858MB against 15GB of free space.
• scripts/setup-worktree.sh creates a worktree and populates node_modules with cp -c instead, which on APFS clones it copy-on-write: blocks stay shared until npm rewrites a file. Measured on a fresh worktree: 5.2s / 8.6MB of real disk, against ~90s / 424MB for npm ci.
• Exposed as npm run setup:worktree -- <name> [branch].

Behavior notes for reviewers:

• New branches are based on origin/master after a git fetch, per the branch rules in CLAUDE.md; an existing branch name is checked out as-is.
• The main checkout is resolved via --git-common-dir, so the script works when invoked from inside another worktree (where .git is a file).
• npm install reruns only when the branch's package-lock.json differs from the main checkout's. Note that npm ci would defeat the clone entirely, since it deletes node_modules before reinstalling.

Test plan

• Ran the script end to end, then npm test inside the created worktree — 48 tests passed, so the cloned node_modules is functional.
• Verified workspace symlinks (autonyan-shared -> ../src/shared) and node_modules/.bin/* survive the clone as relative links, and that require.resolve('autonyan-shared') resolves into the new worktree rather than the main checkout.
• Measured real disk consumption via df before/after (8.6MB) rather than du, which reports the full 434MB for CoW clones.
• npm run lint, the read-only formatting checks, and npm run test:coverage all pass. shfmt -d scripts/*.sh was verified separately via go run since shfmt is not installed locally; tflint could not be run locally either, but this PR changes no Terraform.
• Removed the verification worktree and its branch afterwards.

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[kkohtaka]

Closing: this addresses the wrong code path.

The worktrees that actually fill the disk are created by Claude Code itself — there are 41 claude/* branches locally, each Created from refs/remotes/origin/master by the harness. npm run setup:worktree is a manual entry point that never sits in that path, so it would not have prevented any of the accumulation it was written for. It also does nothing about the accumulation itself; the cleanup in this session was done by hand.

The measurement stands (cp -c clones node_modules for ~8.6MB / 5.2s instead of ~424MB / 90s on APFS), but the useful primitive is "populate node_modules cheaply in the worktree we are already in", applied automatically, rather than "create a worktree". Will reopen the work in that shape.