feat(whiteboard): add docs write-back mode

[markphelps]

Adds /whiteboard docs, which carries what a map learned back into the repo's own docs as a reviewable branch.

What it does

• Inputs: stale-doc, undocumented, and confusing friction, [stated] rationale, unexplained decisions, and refuted suspected bugs.
• Targets: README, ARCHITECTURE.md, CONTRIBUTING, docs/, AGENTS.md/CLAUDE.md, and code comments/docstrings. It follows existing repo docs conventions and never proposes code renames. ADR-shaped items are handed off by wording ("candidates for decision records (ADRs)"), with a design-decisions doc as the fallback.
• Flow: check whether the map is tracked (ask once to include it or write self-contained docs, recorded in .map/scout.md) → propose changes in chat, grouped by file, and the user picks → write in a git worktree branch as two commits (docs, then comments-only) → show the diff stat and push/open a PR only on an explicit yes.
• Comment check: the comments commit is rejected if any changed line isn't entirely a comment or docstring.
• Content rules: stale claims are corrected in place. Missing docs get a short summary plus a map link when the map is shared, or fuller self-contained sections when it isn't. [inferred] rationale never goes into repo docs. PR evidence cites code receipts and commits, never map paths.
• Map lifecycle: included entries are marked proposed: <branch>. On the next /whiteboard or refresh, landed changes remove the entry and upgrade evidence to [documented: <path>]; abandoned branches reopen it.

Other whiteboard changes

• The hard rule changes from "never modify source code" to "never modify the working tree", with docs as the only, narrowly scoped exception.
• New friction types stale-doc and undocumented. inconsistent now covers only "two patterns doing the same job".

Checks

Ran locally: npm run format:check, npm run validate, and npm run sync (mirror under skills/ included).