docs: Phase 21 spec — point-and-tell feedback

[vicmaster]

What

Planning artifact only (no code): spec for Phase 21 — point-and-tell feedback, plus the VISION.md phase entry.

The feature: the human clicks any element in the viewer, types a note, and it lands as structured feedback ({ nodeId, comment } on canvas.metadata.feedback[]) that the agent reads via a new get_feedback tool and closes via resolve_feedback. The inverse of canvas_evaluate — human critique becomes machine-readable, ending the "the third card, no the OTHER one" prose loop.

Why it's cheap for what it buys

Verified against current src/ — every hard piece already exists:

• renderer stamps data-node-id on every element (renderer.ts:233); detail page iframes are same-origin and Phase 19's highlightNode already reaches into them
• repo write-back path exists (archiveRepoCanvas pattern in aggregate.ts) — spec extracts a generic updateRepoCanvas
• ensureFresh mtime reload delivers viewer-written comments to the running MCP server with zero new plumbing

Review focus — two ★ forks

• C1 storage: on-canvas metadata.feedback[] (recommended) vs separate feedback.json
• C2 tool surface: two tools get_feedback/resolve_feedback (recommended) vs one action-enum tool

Everything else has a recommended default a review comment can flip. Slices: A (model/core/tools) → B (viewer UI) → C (loop integration).