feat: add project docs (DOC#) for non-repo knowledge

[gf5901]

Summary

• Adds a DOC#<slug> DynamoDB record type under PROJECT#<id> for storing non-repo knowledge (credentials inventory, infra references, business context) that agents need but shouldn't live in the target repo
• Full CRUD across all layers: Python helpers, ./ctx docs CLI, Hono Lambda API routes, and a Docs tab on ProjectDetail in the frontend UI
• Slug validated with ^[a-z0-9][a-z0-9_-]{0,62}$ on both server and client; content capped at 50KB per doc

Changes

Python (src/projects_dynamo.py)

• list_docs, get_doc, put_doc, delete_doc with slug validation

CLI (src/context_cli.py)

• ./ctx docs — list all docs with slug, title, preview
• ./ctx docs <slug> — print full doc content

Hono API (infra/packages/api/)

• GET /projects/:id/docs — list all
• GET /projects/:id/docs/:slug — get one
• PUT /projects/:id/docs/:slug — create or update (idempotent)
• DELETE /projects/:id/docs/:slug — remove

Frontend (frontend/src/pages/ProjectDetail.tsx)

• New "Docs" tab with create form (slug + title + markdown content), inline editing, delete with confirmation, markdown rendering

Docs

• docs/dynamo-schema.md — DOC# record type
• AGENTS.md and project conventions updated

Test plan

• Create a project, navigate to the Docs tab
• Create a new doc with slug aws-setup, verify it appears in the list
• Edit the doc content, verify the update persists after reload
• Delete the doc, verify it's removed
• Verify ./ctx docs and ./ctx docs aws-setup work from an agent temp dir
• Verify deleting a project also cleans up its docs (existing deleteProject queries all items under the pk)
• Verify slug validation rejects invalid slugs (leading hyphens, special chars, > 63 chars)

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