fix(mosaic): close claudex isolation gaps S1 (Bedrock/Vertex bypass) + S2 (fail-open catches)

Addresses MS-LEAD secrev REQUEST CHANGES on #806.

S1 (CRITICAL — REQ 2 defeated by provider switches + cloud creds): the env sweep
did not neutralize AWS Bedrock / GCP Vertex routing. CLAUDE_CODE_USE_BEDROCK /
CLAUDE_CODE_USE_VERTEX are routing switches whose mere presence makes Claude Code
talk to the real Anthropic API via the ambient cloud credential chain, bypassing
ANTHROPIC_BASE_URL (the loopback proxy) entirely — and the cloud credentials
(AWS_SECRET_ACCESS_KEY, AWS_BEARER_TOKEN_BEDROCK, GOOGLE_APPLICATION_CREDENTIALS,
…) passed straight through.
- buildClaudexEnv now force-deletes the routing switches by exact name
  (CLAUDEX_FORCED_UNSET_ENV) regardless of value — a name pattern is the wrong
  model for a boolean switch.
- CLAUDEX_CREDENTIAL_ENV_RE extended to the cloud-cred families (^AWS_,
  ^GOOGLE_APPLICATION_CREDENTIALS, ^GOOGLE_CLOUD_, ^GCP_) and the mid-string
  _KEY$ / _SECRET gap (STRIPE_SECRET_KEY, SSH_PRIVATE_KEY, AWS_SECRET_ACCESS_KEY).
- Doc-comment updated to match reality.

S2 (HIGH — fail-open contradicts documented fail-closed): defaultCanonicalizeIntended
and defaultIsSymlink swallowed ANY fs error. Now they distinguish ENOENT
(genuinely absent → safe to continue) from every other errno (ELOOP, EACCES,
ENOTDIR, …) which rethrows and fails CLOSED.

Tests (TDD red-first, 4 new specs red before the fix): Bedrock/Vertex switches +
AWS/GCP creds swept while the loopback proxy stays the only route; mid-string
_KEY/_SECRET closed; real-FS ELOOP (canonicalize) and ENOTDIR (isSymlink) fail
closed; injected EACCES not swallowed.

New-module coverage 100% stmts/lines, 93.75% branch. Full mosaic suite 1169 green;
typecheck / lint / format:check green. No --no-verify.

Refs #790

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/mosaic/README.md

@@ -82,9 +82,14 @@ resolved dir can never be — or live under — the real `~/.claude`. A claudex
 session therefore cannot mutate your normal Claude Code config.
 
 **No token leakage.** claudex never reads the proxy's credential file. Claude
-Code is handed only `ANTHROPIC_AUTH_TOKEN=unused`; the entire credential-bearing
-env family (`ANTHROPIC_*`, `*_TOKEN`, `*_API_KEY`, `*_SECRET`, …) is stripped
-from the composed environment. The proxy holds the real OAuth credential.
+Code is handed only `ANTHROPIC_AUTH_TOKEN=unused` pointed at the loopback proxy;
+the entire credential-bearing env family (`ANTHROPIC_*`, `AWS_*`, `GOOGLE_CLOUD_*`,
+`GOOGLE_APPLICATION_CREDENTIALS`, `*_TOKEN`, `*_KEY`, `*_SECRET`, …) is stripped
+from the composed environment. The Bedrock/Vertex routing switches
+(`CLAUDE_CODE_USE_BEDROCK`, `CLAUDE_CODE_USE_VERTEX`, and the `_SKIP_*_AUTH`
+pair) are force-removed regardless of value — otherwise their mere presence
+would route Claude Code to the real Anthropic API via AWS/GCP and bypass the
+proxy. The proxy holds the real OAuth credential.
 
 **Model tiers (override via env).**