Add real collaborative Cowork editors

[gitcommit90]

Outcome

Cowork now provides real file-backed collaborative editors instead of placeholder surfaces: CodeMirror for Notes, Docs, and Code; embedded Excalidraw for Whiteboards; Excalidraw-backed presentations; and authenticated Yjs presence and synchronization over the existing channel workspace.

The Cowork agent panel still creates an ordinary channel thread. Its first message adds the validated open workspace path and active co-viewers server-side, with no parallel agent harness or document store.

Safety and continuity

• Membership-gated collaboration sockets and supported workspace roots only
• Same-origin self-hosted Excalidraw fonts/assets with a fail-closed build rewrite
• Editor/canvas nodes, focus, cursor, draft, scroll, and presence preserved across shell/theme updates
• External agent/Terminal edits refresh clean documents; stale drafts require confirmation
• Dirty rooms flush during shutdown; deleted or moved paths cannot reconnect stale sockets
• Excalidraw transitive high/moderate advisories resolved with narrow patched overrides

Verification

• npm run typecheck
• npm run build
• npm test: 117 native assertions plus 104 passing contracts, 2 expected default browser-discovery skips
• onboarding browser: 20/20
• brief regression browser: 38/38
• two-user Cowork browser: pass
• feedback browser: pass
• production-data browser: 18/18 against a disposable live-data copy
• real terminal reconnect browser: pass
• mobile browser: 4/4
• site: 5/5
• npm audit --omit=dev: 0 high, 0 critical; 17 pre-existing Spectrum/OpenTelemetry moderate findings whose suggested fix is a breaking Spectrum downgrade
• git diff --check

Versioned as 0.0.18 with release pins, changelog, README, and user guide updated.

Signed-off-by: Joseph Yaksich gitcommit90@users.noreply.github.com