fix(token-saver): secure management routes and honor PXPIPE opt-out

[KunN-21]

Summary

• restrict PXPIPE and spawn-capable Headroom management routes to the existing local-only trust boundary
• make X-9Router-Token-Saver: off disable PXPIPE as well as RTK, Headroom, Caveman, and Ponytail
• replace the previous false-positive PXPIPE bypass coverage with a real Claude-format, above-threshold negative/positive control

Security context

When dashboard login is disabled, generic /api/* routes are intentionally available remotely. Child-process/package-manager routes remain local-only. This PR extends that existing boundary to:

• /api/pxpipe/*
• /api/headroom/extras
• /api/headroom/restart

PXPIPE installs a hardcoded pxpipe-proxy@latest package; this is a package-manager execution boundary, not attacker-controlled package selection. Headroom extras are also constrained to a closed code/ml whitelist.

This PR supersedes #3078 by including its /api/pxpipe guard plus Headroom coverage and focused tests. It does not close #3078 automatically.

Behavior

• remote requests to these management prefixes receive 403, even when requireLogin=false
• loopback authenticated/login-disabled access and valid CLI-token access remain allowed
• X-9Router-Token-Saver: off or OFF returns PXPIPE skip telemetry (applied:false, reason:"disabled") and leaves the request body unchanged
• no header keeps current PXPIPE behavior

Test plan

• unit/dashboard-guard.test.js
• unit/headroom-chat-core.test.js
• unit/pxpipe.test.js
• 46 tests passed, 0 failed
• no-regression baseline: 0 new failures
• provider, alias, and OAuth snapshots unchanged

Scope

Exactly four files. No dependency, version, changelog, Headroom pipeline, or system-injection changes.

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[KunN-21]

Bảo trì: đã merge upstream eb712ca và push thường tip 8400a34. Diff còn 4 file, giữ local-only guards và PXPIPE opt-out. Review độc lập không thấy blocker; 39/39 scoped tests pass. Cohort liên quan có 1 lỗi headroom-detect Windows path, đã tái hiện trên upstream cùng SHA; không sửa lỗi ngoài phạm vi trong PR này. git diff --check pass. Chưa chạy full suite hoặc live provider.