feat(herdr): add fail-closed terminal runtime integration

[devswha]

Summary

• add a runtime-neutral terminal contract alongside the existing tmux adapter
• add opt-in Herdr v0.7.5 discovery, output, and no-takeover control
• enforce source, policy, principal, identity-generation, frame, queue, and teardown bounds
• expose strict discovery v2 and shell v3 protocols without using Herdr for provider/GJC/create flows
• add the hardened Phase 0 probe, sanitized fixture, live/adversarial reports, and self-hosting guidance

Safety boundaries

• supports only the independently measured Herdr v0.7.5/protocol 17 cohort
• Herdr v0.8.2 and every unreviewed fingerprint fail closed
• ChatMux never installs, starts, upgrades, repairs, removes, or takes over Herdr
• generic Herdr panes stay attach-only and never gain action authority

Verification

• official v0.7.5 x86_64 asset SHA-256 rechecked: 3dc83288073e4c2d3c679a30e7be97bcca9141c6fd17dbbb9219142e95c59253
• npm run verify (audit, typecheck, Rust fmt/clippy/tests, server/client tests, lint, identity, production build)
• refreshed against current main, then reran typecheck and shell/image/Herdr protocol tests

Review gate

Draft until the independent architecture/security review required by docs/HERDR-PHASE0.md approves the pinned cohort. The current upstream latest is v0.8.2 and is deliberately unsupported pending a fresh Phase 0 run.

[devswha]

Deferred to #58. The implementation branch and passing CI evidence are preserved; this draft can be restored or replaced after the compatibility and independent review gates are satisfied.