fix(security): IPv6 ULA blocking, cookie redaction, per-tab cancel, targeted token (#664)

Community PR #664 by @mr-k-man (security audit round 1, new parts only).

- IPv6 ULA prefix blocking (fc00::/7) in url-validation.ts with false-positive
  guard for hostnames like fd.example.com
- Cookie value redaction for tokens, API keys, JWTs in browse cookies command
- Per-tab cancel files in killAgent() replacing broken global kill-signal
- design/serve.ts: realpathSync upgrade prevents symlink bypass in /api/reload
- extension: targeted getToken handler replaces token-in-health-broadcast
- Supabase migration 003: column-level GRANT restricts anon UPDATE scope
- Telemetry sync: upsert error logging
- 10 new tests for IPv6, cookie redaction, DNS rebinding, path traversal

Co-Authored-By: mr-k-man <mr-k-man@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

supabase/migrations/003_installations_upsert_policy.sql

@@ -0,0 +1,25 @@
+-- 003_installations_upsert_policy.sql
+-- Re-add a scoped UPDATE policy for installations so the telemetry-ingest
+-- edge function can upsert (update last_seen) using the caller's anon key
+-- instead of the service role key.
+--
+-- Migration 002 dropped the overly broad "anon_update_last_seen" policy
+-- (which allowed UPDATE on ALL columns). This replacement uses:
+--   1. An RLS policy to allow UPDATE (required for any row access)
+--   2. Column-level GRANT to restrict anon to only the tracking columns
+--      the edge function actually writes (last_seen, gstack_version, os)
+--
+-- This means anon callers cannot UPDATE first_seen or installation_id,
+-- closing the residual risk from the broad RLS-only approach.
+
+-- RLS policy: allow UPDATE on rows (required for PostgREST/upsert)
+CREATE POLICY "anon_update_tracking" ON installations
+  FOR UPDATE
+  USING (true)
+  WITH CHECK (true);
+
+-- Column-level restriction: anon can only UPDATE these three columns.
+-- PostgreSQL GRANT UPDATE (col, ...) is enforced at the query level —
+-- any UPDATE touching other columns will be rejected with a permission error.
+REVOKE UPDATE ON installations FROM anon;
+GRANT UPDATE (last_seen, gstack_version, os) ON installations TO anon;