Hardening GitHub Workflow pathing and documentation

Context: Discovered during the review of PR #11149 (which resolved process.cwd() leakage in GitHub Workflow pathing).

The Problem: The config.projectRoot resolution logic currently handles the canonical neomjs/neo repository and standard npx neo-app node_modules installations. However, three non-blocking edge cases and documentation gaps were identified that should be resolved to guarantee robust execution:

1. pnpm installs (.pnpm/neo.mjs@*/node_modules/neo.mjs/) have a distinct directory layout edge case that is currently unhandled by the path heuristic.
2. The newly introduced NEO_WORKSPACE_ROOT manual override environment variable is undocumented and needs to be surfaced in user-facing configuration docs.
3. The defaultBranchDetector in toolService.mjs was updated to be cwd-aware, but currently lacks a dedicated Cycle-2 unit test to prevent future regressions.

The Architectural Reality:

• ai/mcp/server/github-workflow/config.mjs and config.template.mjs
• ai/services/github-workflow/toolService.mjs
• Relevant user-facing markdown documentation for MCP configurations.

The Fix:

• Enhance the projectRoot heuristic to support pnpm workspace layout matching.
• Document NEO_WORKSPACE_ROOT in the relevant markdown files.
• Add a Playwright unit test specifically validating defaultBranchDetector with a mock cwd.

Acceptance Criteria:

• projectRoot heuristic successfully detects pnpm layout without breaking existing npm/yarn layouts.
• NEO_WORKSPACE_ROOT is documented.
• defaultBranchDetector unit test added and passing.

Out of Scope:

• Modifying other MCP servers' path resolution logic.

Related:

• PR #11149

Origin Session ID: e93e8f88-2d80-4a24-80c7-7a366d10651b