Docs: cover the per-agent whiteboard (#752 shipped with no docs)

[selfcontained]

Nightly docs audit — 2026-07-28

Slice audited: diff-driven. PR #752 (per-agent Excalidraw whiteboard, v0.31.0) shipped with an ambient tip but zero coverage in the in-app docs, README, or API spec — the "whole new surface, missing section" failure mode.

What changed

• docs-pane Agents: new Whiteboard subsection (agents#whiteboard) — shared canvas, live two-way sync with element-id merge, the three whiteboard_* MCP tools, the browser-rendered PNG snapshot caveat, and the violet agent-drew dot. Split-pane copy updated to include the Whiteboard tab (all three tabs are draggable into splits).
• docs-pane Repo Tools: whiteboard_get / whiteboard_update / whiteboard_clear added to the built-in tools list.
• docs-pane Reviewers: notes reviewers get read-only whiteboard_get for their own board (verified against REVIEW_AGENT_TOOLS and the MCP context wiring — it is not the parent's board).
• tips.ts: whiteboard tip now deep-links to agents#whiteboard.
• README: whiteboard feature bullet; three rows in the interactive-agents MCP table; corrected the persona tool list (was also missing dispatch_delete_pin, dispatch_list_pins, dispatch_list_media, dispatch_delete_media).
• docs/03-api-spec.md: new Whiteboard section (4 REST routes, 20k-element / 8 MB caps, snapshot file location) + whiteboard.changed row in the SSE event table.

Deferred to next run: the Automations (Templates & Jobs) deep-dive that was this run's original next_focus — the whiteboard gap took priority per the diff-first rule.

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