feat: gr2 shared doc scratchpads MVP

[laynepenney]

Summary

Build the first production slice of shared scratchpads as a doc-first collaborative workspace surface.

Why

The shared scratchpad prototype showed that the immediate workflow pain is lightweight collaboration on documents and notes, not general shared coding. A doc-first slice solves the real problem without weakening private implementation lanes.

Scope

First production slice should support:

• create a shared doc scratchpad
• list shared doc scratchpads
• show shared doc scratchpad metadata
• track purpose, participants, refs, and lifecycle
• scaffold docs/notes/context roots under a shared workspace-owned location

Acceptance criteria

• shared doc scratchpads are clearly separate from private lanes
• metadata is explicit and legible
• users can collaborate on docs/blog/release-note content without entering another worker's private directory
• the surface is lightweight enough to use before a PR exists

Out of scope

• general shared coding lanes
• background execution
• replacing PR review

Design refs

• design: shared scratchpads for lightweight collaborative work #552
• docs/PLAN-gr2-shared-scratchpads.md

[laynepenney]

This issue is the build-side follow-on from the verified shared scratchpad prototype in #552.

Planned scope for the first production slice:

• doc-first shared scratchpads
• explicit metadata
• list/show/create surfaces
• no general shared coding lane in the first pass

[laynepenney]

MVP note: #553 should be treated as blocked on the prototype stress matrix, not just the happy path.

Reference:

• docs/ASSESS-gr2-shared-scratchpads-stress.md

The first production slice should only move once we are satisfied with the answers to the break scenarios there.