fix(extension): strip webview zoom bridge and diag lanes (amicode#266)

[gennadiryan]

What / why

amicode#266 — Cmd/Ctrl +/-/0 zoom keybindings did nothing while focus was inside the chat webview. The fix moves zoom handling out of the extension entirely: the vendored fork now captures the chords in each app document and applies its in-app CSS zoom (companion PR in harmoniqs/opencode).

This PR removes the extension-side machinery built for the previous capture-and-forward approach:

• chat_bridge.ts — deleted the zoom lane, the diag-log lane, the eager "Amicode — webview diag" output channel and its activation probe.
• chat_panel.ts — deleted the host-page keydown listener and the zoom / diag-log allowlist entries.
• tests + __mocks__/vscode.ts — removed the zoom/diag test blocks and the outputLines channel mock.

Why the previous approach could not work: keydowns inside the app iframe never reach the workbench keybinding service (cross-origin, frame-local event routing), so no extension-side capture point existed for panel focus; with the webview tab focused, host-page capture double-fired alongside the workbench's own native zoom (preventDefault cannot suppress the webview's keydown forwarding).

Verification

• 17/17 chat_bridge tests; full fast suite green apart from environment-dependent runs/ledger tests (need amico / ~/.amico); tsc --noEmit clean.
• Remote GUI: panel focus → single CSS zoom of the webview content; tab focus → single native window zoom; no doubles, no diag channel.

References #266.