[agent] Add a read-only local Agent Core doctor

[sam2tom]

Context

Connection failures currently require operators to manually inspect a Core URL, proxy configuration, caller key files, daemon state, and provider setup. A safe local diagnostic command can make those layers visible without modifying Core state or sending a paid Turn.

Observable outcome: contributors can run one read-only command and receive a redacted, layered diagnosis instead of copying secrets or guessing from /healthz.

Acceptance criteria

• Add pnpm core:doctor with an optional explicit Parsar checkout path.
• Check the configured proxy target and conventional token-file path without printing token contents.
• When local files are available, validate file presence/permissions and compare the caller token digest with the matching keys.json binding in memory only.
• Check Core network liveness and authenticated basic Agents API reads without creating or changing any resource.
• Detect non-sensitive daemon profile/process status when the upstream CLI exposes it.
• Report separate states such as Core unreachable, Core API authenticated, daemon not observed, and provider not verified; never collapse them into a misleading Ready.
• Treat execution/model readiness as Unknown unless a stable upstream contract proves it.
• Redact tokens, authorization headers, URL query/fragment data, provider credentials, Session content, and private paths from shareable output.
• Exit nonzero for actionable configuration failures and document the exit-code contract.
• Add fixture-driven tests for missing files, unsafe permissions, digest mismatch, unreachable Core, 401, authenticated API, absent daemon, and redaction.

File ownership

• new scripts/core-doctor.mjs
• new scripts/core-doctor.test.mjs and scripts/fixtures/core-doctor/**
• package.json only for the command/check entry

Do not edit connection UI, App.tsx, README files, or Agents client protocol types.

Validation

• Run the doctor against fixtures and a deliberately unreachable loopback port.
• Verify stdout/stderr, argv, fixtures, and snapshots contain no secret value.
• Run pnpm check and git diff --check.
• No database write, Session/Turn creation, or paid model request is allowed.

Non-goals

• Do not generate or rotate credentials.
• Do not run migrations or manage Core/daemon processes.
• Do not claim provider readiness from process existence.
• Do not implement Parsar Core behavior in this repository.