bootstrapWorktree CLI mis-resolves projectRoot one level short

Context

Surfaced during #12145 worktree setup: running node ai/scripts/migrations/bootstrapWorktree.mjs left the worktree's config.mjs files missing (forcing a manual template-copy fallback to run unit tests) and created a stray ai/ai/ directory containing exactly the BOOTSTRAP_CONFIGS set.

The Problem

The CLI entry (isMain block) computes the repo root one directory-level short:

const projectRoot = path.resolve(__dirname, '..', '..'); // ai/scripts/ → ai/ → root

The script lives at ai/scripts/migrations/bootstrapWorktree.mjs (3 levels deep), so __dirname is <root>/ai/scripts/migrations. Two .. climbs only to <root>/ai, not the repo root. The comment ("ai/scripts/ → ai/ → root") is stale — it describes the pre-migrations/ location; the file was relocated into migrations/ without bumping the .. count. The config copy then does path.join('<root>/ai', 'ai/config.mjs') → writes to <root>/ai/ai/config.mjs, so the real ai/config.mjs never lands and every consumer that imports ai/services.mjs hits ERR_MODULE_NOT_FOUND.

The Architectural Reality

• ai/scripts/migrations/bootstrapWorktree.mjs:901 — the buggy path.resolve(__dirname, '..', '..').
• The exported bootstrapWorktree({projectRoot}) takes projectRoot as an argument, so the unit spec injects it correctly — the CLI-only inline computation is the untested glue. test/playwright/unit/ai/scripts/migrations/bootstrapWorktree.spec.mjs's JSDoc explicitly states the CLI path is "a thin wrapper ... not exercised here," which is how the off-by-one escaped coverage (same escape class as #12042).

The Fix

1. Extract the CLI root computation to an exported, testable helper resolveCliProjectRoot(moduleDir) returning path.resolve(moduleDir, '..', '..', '..') (migrations/ → scripts/ → ai/ → root), with a corrected comment.
2. Use it in the isMain block: const projectRoot = resolveCliProjectRoot(__dirname);.
3. Add a spec case asserting it resolves <root>/ai/scripts/migrations → <root> (with a regression note that the prior 2-level resolve mislanded at <root>/ai).

Acceptance Criteria

• AC1: resolveCliProjectRoot('<x>/ai/scripts/migrations') returns <x> (repo root), unit-tested.
• AC2: the isMain CLI path uses the helper; no inline 2-level path.resolve(__dirname, '..', '..') remains.
• AC3: the stale comment is corrected.
• AC4: the bootstrapWorktree spec passes.

Out of Scope

• The build-all step's separate failure modes (heavy full build) — orthogonal to the path bug.
• A broader CLI-path integration test — the helper unit test covers the escape class.

Related

• Surfaced during #12145 (pull-mode tenantRepos tiered resolver) worktree setup.
• Bootstrap feature origin: #10095; the relocation into migrations/ likely introduced the regression.

Decision Record impact

none — localized bug fix.

Handoff Retrieval Hints

• Retrieval Hint: "bootstrapWorktree projectRoot off-by-one ai/ai stray configs"