fix(cli): revert detect-squad-dir to zero-dependency bootstrap

[diberry]

Bug

detect-squad-dir.ts line 5 — the requested module @bradygaster/squad-sdk does not provide an export named FSStorageProvider

Reported on the insider branch; same code exists on dev.

Root Cause

Commit 26047dc5 (feat(storage): add StorageProvider abstraction layer) swept the codebase converting raw node:fs calls to FSStorageProvider. It incorrectly converted detect-squad-dir.ts, a bootstrap utility whose JSDoc explicitly says "zero dependencies". This file runs to detect .squad/ directories before the SDK is loaded — it must only use Node built-ins.

Fix

• Reverted detect-squad-dir.ts to use raw node:fs (fs.statSync, fs.readFileSync, fs.existsSync)
• Removed import { FSStorageProvider } and const storage = new FSStorageProvider()
• Preserved the "zero dependencies" JSDoc banner

Regression Guard

Added test/detect-squad-dir-zero-deps.test.ts with three assertions:

1. Source must not import from @bradygaster/squad-sdk
2. Source must not require @bradygaster/squad-sdk
3. Source must import from node:fs

This prevents future sweeping refactors from accidentally converting the file again.

Verification

• npm run build ✅
• vitest run test/worktree.test.ts — 9/9 pass ✅
• vitest run test/detect-squad-dir-zero-deps.test.ts — 3/3 pass ✅

[Copilot]

Pull request overview

This PR fixes a CLI startup regression by restoring detect-squad-dir.ts to a true zero-dependency bootstrap utility (Node built-ins only), and adds a regression test + updated contributor guidance to prevent future refactors from reintroducing SDK dependencies.

Changes:

• Reverted packages/squad-cli/src/cli/core/detect-squad-dir.ts to use node:fs directly instead of FSStorageProvider.
• Added test/detect-squad-dir-zero-deps.test.ts to enforce “no SDK imports/requires” and “must use node:fs”.
• Documented a “Protected Files — Zero External Dependencies” list in .github/copilot-instructions.md, plus added a changeset.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
packages/squad-cli/src/cli/core/detect-squad-dir.ts	Removes SDK dependency and restores raw node:fs usage for bootstrap safety.
test/detect-squad-dir-zero-deps.test.ts	Adds a regression guard to prevent reintroducing external deps into the bootstrap file.
.github/copilot-instructions.md	Documents protected bootstrap files and refactor rules to avoid repeating the regression.
.changeset/fix-detect-squad-dir-zero-deps.md	Records a patch-level release note for the CLI fix.