feat(server,agent): v2 secure-session-core Task 5 - attended consent

SPEC-002 Phase 1 Task 5, code-reviewed APPROVED. An attended (support-code)
session is invisible and inert to the technician until the end user accepts a
consent prompt on their own machine.

- proto: ConsentRequest / ConsentResponse + ConsentAccessMode enum (oneof
  fields 80/81; no existing field renumbered).
- server: ConsentState on Session; attended -> Pending, managed -> NotRequired;
  join_session refuses viewers unless Granted/NotRequired (single chokepoint -
  StartStream only fires from join_session, so no frames or input flow pre-
  consent); run_consent_handshake sends ConsentRequest, 60s timeout, granted ->
  proceed, denied/timeout/disconnect -> teardown (end_session denied, machine
  offline, support code released). consent_state persisted; consent_requested/
  granted/denied audited.
- agent: Windows MessageBox (topmost/system-modal) on spawn_blocking; anything
  but an explicit Yes = deny; non-Windows build is a fail-closed stub.

Not cargo-check-verified locally (no toolchain). Server verified on the build
host; the Windows agent half is verified by CI build-agent (Pluto).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

agent/src/main.rs

@@ -15,6 +15,7 @@
 mod capture;
 mod chat;
 mod config;
+mod consent;
 mod encoder;
 mod input;
 mod install;