docs: decide signed Agent Card architecture

[KenTaniguchi-R]

Summary

• choose immutable-per-revision, CLI-signed public/default Agent Cards plus a separate authenticated viewer authorization extension
• bind Ed25519 JWS verification to the expected host and handle through exact subject-scoped JWKS discovery
• define delegated signer registration, proof of possession, rotation, expiry, rename, recovery, storage, and zero-user cutover semantics
• preserve employee-card non-enumeration and state the same-relay host-authorized trust boundary without TOFU or malicious-relay overclaims
• add the decision to the living reference index

Validation

• pnpm -r build
• pnpm -r typecheck
• pnpm -r test (1,121 passed; 3 expected platform probes skipped)
• two independent architecture/security reviews: SHIP

Closes #101

[KenTaniguchi-R]

CI is externally blocked before runner allocation: both required GitHub Actions jobs completed with zero steps, and both annotations report that the jobs were not started because of failed account payments or an Actions spending limit. GitGuardian passed. Local required gates are green (build, typecheck, and 1,121 tests), and two independent reviews returned SHIP. I am leaving this PR unmerged until the billing/spending-limit issue is fixed and required checks can run.