chore: document docs repo worktree hygiene

[Delqhi]

Summary

• closes chore: document nested worktree hygiene for the docs repo #146
• document where companion repos and local worktrees belong so the docs repo root stays clean
• exclude nested worktree paths from docs CI scans and repo-health workflows
• add a fail-fast worktree hygiene script before docs dev/build entry points
• tell the VitePress dev watcher to ignore nested git/worktree paths that can cause recursive scan loops

Validation

• workflow YAML parses with python3 + yaml.safe_load
• package.json parses successfully with python3
• bun ./scripts/worktree-hygiene.mjs now fails fast when nested .git artifacts exist inside the repo root (expected on a dirty local workspace)

[Delqhi]

Worktree hygiene now covers three layers in this branch:

• docs and repo-health workflows ignore nested worktree paths
• predev / prebuild run a fail-fast nested .git check
• VitePress watch ignores nested git/worktree directories

Follow-up for local environments: nested companion repos still need to be moved to a sibling directory such as ../OpenSIN-worktrees/ so the new hygiene check passes during normal dev / build runs.

[Delqhi]

Post-relocation verification update:

• bun ./scripts/worktree-hygiene.mjs now passes locally
• bun ./scripts/build-docs.mjs now passes locally
• GIT_MASTER=1 git status --short --branch is clean on chore/docs-worktree-hygiene-146
• coder-SIN-Qwen discovery still resolves after the move at /Users/jeremy/dev/OpenSIN-worktrees/coder-SIN-Qwen

The local nested repos/worktrees were moved to /Users/jeremy/dev/OpenSIN-worktrees/, so the new Git / CI / VitePress hygiene guard is now validated end-to-end.

[Delqhi]

Launch-sequencing note: PR #151 is now open as the repo-side mitigation for blocker #150. Recommended merge order is #151 -> #149 -> #148, followed by a production deploy from main and a live smoke on docs.opensin.ai.