docs: add plans/README.md documenting the plan-document convention (#753)

[chethanuk]

📋 Summary

plans/ has 16 directories and no top-level file explaining what they are, so the only written
statement of the convention is one clause in CONTRIBUTING.md ("create a plan document at
plans/<issue-number>/") — already narrower than the tree it governs, and never mentioned in
AGENTS.md, the agent entry point. Adds plans/README.md documenting the structure as it
actually exists, and points AGENTS.md at it.

🔗 Related Issue

Closes #753

🔄 Changes

• Add plans/README.md: what the directory is (development planning artifacts, never
  published to the docs site), the plans/ vs architecture/ vs fern/ vs docs/ boundary,
  both live naming conventions (plans/<issue-number>/ and plans/<workstream-name>/), the
  document shape as observed, where assets go, and how a plans-only PR is reviewed per
  .agents/recipes/pr-review/recipe.md.
• Add one line to AGENTS.md pointing at plans/, matching the three sentences already there.
• CONTRIBUTING.md is intentionally unchanged — GitHub renders a directory's README.md, so
  its existing relative link (and the Fern copy's absolute one) already land on the new file.

🧪 Testing

• make test — N/A, documentation-only change
• Unit tests added/updated — N/A, no executable path in this diff; plans/ has no test
  coverage and the repo has no markdown linter
• E2E tests added/updated — N/A
• uvx pre-commit run --files plans/README.md AGENTS.md passes (trailing-whitespace,
  end-of-file-fixer, mixed-line-ending)
• Every relative link in plans/README.md resolved by hand against git ls-tree origin/main plans/

✅ Checklist

• Follows commit message conventions
• Commits are signed off (DCO)
• Architecture docs updated — N/A, this change doesn't touch architecture/

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[github-actions]

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[greptile-apps]

Greptile Summary

This documentation-only PR establishes a top-level guide for development plans and makes it discoverable from the agent entry point.

• Documents the boundaries between planning artifacts, architecture references, published Fern content, and documentation support files.
• Describes existing naming, document, asset, and lifecycle conventions.
• Accurately summarizes the repository’s plans-only review process.

Confidence Score: 5/5

The PR appears safe to merge with no actionable correctness, documentation, or repository-rule issues identified.

The documented conventions agree with the cited repository files and workflows, and all added links resolve.

Important Files Changed

Filename	Overview
AGENTS.md	Adds a valid link directing development agents to the new planning convention.
plans/README.md	Documents repository planning conventions with accurate paths, examples, workflow behavior, and review guidance.

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