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>

CLAUDE.md

@@ -429,9 +429,16 @@ leak always BLOCKs (deterministic).
 - Classifier model cache: `~/.gstack/models/testsavant-small/` (112MB, first run only)
 - Attack log: `~/.gstack/security/attempts.jsonl` — written by
   `tunnel-denial-log.ts` (tunnel-surface rejections; rotates at 10MB, 5 generations)
-- Session state: `~/.gstack/security/session-state.json` (cross-process, atomic;
-  NOTE: classifierStatus currently has no live writer — shield status derives
-  from what's on disk)
+
+History note (#2557): the cross-process session state
+(`~/.gstack/security/session-state.json`), `getStatus()`, the `/health`
+`security` field, and the sidepanel SEC shield were all removed — the state
+file lost its only writer when sidebar-agent.ts was ripped, so the shield
+reported a permanent 'inactive' or a stale false-green 'protected' from
+leftover disk state. The live defenses (L1-L3 filters, L4 sidecar on the
+inject-scan path) report through their own call sites, never through
+/health. `browse/test/server-security-surface.test.ts` pins both the
+removal and the live L4 wiring. Do not re-document these as live.
 
 ## Dev symlink awareness