Add policy pack: docs-writer (read code, edit markdown)

[saivedant169]

Problem

We ship three policy packs in starter-kit/policies/: readonly, pr-writer, and infra-review. A common use case that is not well served is an agent that is allowed to rewrite documentation but should not touch code.

Acceptance criteria

• New file: starter-kit/policies/docs-writer.yaml
• Header comment explains when to use this pack
• Allows: read anything, edit *.md, *.mdx, docs/**, README*, CHANGELOG*
• Reviews: create/update PRs on docs files
• Blocks: any write to source files (.go, .py, .js, .ts, etc.), any destructive shell command, any SQL write
• Tuning notes file at starter-kit/policies/docs-writer-tuning-notes.md briefly explains the philosophy
• The main starter-kit/README.md lists the new pack

Files to create or touch

• starter-kit/policies/docs-writer.yaml (new)
• starter-kit/policies/docs-writer-tuning-notes.md (new)
• starter-kit/README.md (add the pack to the list)
• cmd/aegisctl/policypack.go (if the pack list is hardcoded anywhere, add it)

How to test locally

Run the installer with the new pack:

cd starter-kit
./install-pr-writer.sh  # for reference on how the existing one works
# Build an equivalent flow that loads docs-writer.yaml instead

Or just point aegisctl policy-pack show docs-writer at it and verify it parses.

Notes

Look at starter-kit/policies/pr-writer.yaml and starter-kit/policies/pr-writer-tuning-notes.md for the house style. Keep it short and opinionated.