fix(browse): remove the dead security shield + unfed /health.security (re-derived from #2557)

The sidebar's SEC shield has been dead UI since the PTY terminal rewrite:
nothing set its data-status, nothing unhid it, and the /health.security
field behind it read getStatus() off ~/.gstack/security/session-state.json
— a file whose ONLY writer (sidebar-agent.ts) was deleted with the chat
path. /health therefore reported a permanent 'inactive', or a stale
FALSE-GREEN 'protected' wherever an old state file survived on disk (a
single unit-test run was enough to plant one). A green shield sourced from
leftover state reads as "no threats detected" when the real state is "not
measured" — the same fail-open class as #2026.

Removed (dead surfaces only): the shield markup/CSS and the stale
sidepanel.js comment; the /health security field and server.ts's getStatus
import; getStatus / SecurityStatus / StatusDetail / SessionState /
read+writeSessionState (and security.ts's dead child_process import); the
session-state + getStatus unit tests — including the round-trip test that
wrote real fixture data into ~/.gstack and left /health green forever.
(The PR's security-sidepanel-dom.test.ts deletion already happened on main
via #2230; its resolveDisconnectCause guard landed via the #2085 typeof
fix. Neither re-applied.)

Kept, per ENG-OV9 — security.ts has LIVE consumers: the pure combiner
(combineVerdict + THRESHOLDS), canary utilities, and extractDomain stay;
server.ts's /pty-inject-scan L4 path (isSidecarAvailable + scanWithSidecar)
is untouched. browse/test/server-security-surface.test.ts pins BOTH
directions: the dead surface stays dead (no /health security field, no
getStatus import, no reader of the security session-state file, shield
markup gone) and the live half stays live (sidecar wiring in server.ts,
combiner/canary exports in security.ts, /health carries no token — the
v1.63 regression wall). A future re-feed from LIVE signals must update
that test deliberately rather than resurrect the state-file path.

F13 (same commit): CLAUDE.md's Sidebar security stack section, ARCHITECTURE.md's
prompt-injection Visibility + critical-constraint paragraphs, and
BROWSER.md's security section now describe the removed surfaces as history,
not live features.

Net -166 lines. Tests: server-security-surface + security +
security-adversarial(+fixes) + security-integration + server-auth 114 pass;
sidepanel-* + extension-token + extension-sender-auth 58 pass / 2 skip.

Re-derived from PR #2557 by @frederik-kaster-noygear.

Co-authored-by: Frederik Kaster <frederik.kaster@noygear.ai>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

BROWSER.md

@@ -880,10 +880,11 @@ sidebar chat pipeline that hosted them. **Canary leak always BLOCKs
 - Attack log: `~/.gstack/security/attempts.jsonl` (salted SHA-256 + domain
   only, rotates at 10MB, 5 generations).
 - Per-device salt: `~/.gstack/security/device-salt` (0600).
-- Session state: `~/.gstack/security/session-state.json` (cross-process,
-  atomic).
 
-A shield icon in the sidebar header shows the live status. See
+There is no security status indicator in the sidebar and no `security`
+field on `/health` (#2557): the session-state file that fed them lost its
+only writer when the chat-path agent was removed, so they reported stale or
+empty data. The live defenses report through their own call sites. See
 ARCHITECTURE.md § "Prompt injection defense" for the full threat model.
 
 ---