PR #2080 — Doctor checks config.json and policy.json without a JSON Schema library

The Broader Goal at the End of the Previous PR

v0.24.0 still needs dispatch, review, and local config to fail in the open instead of drifting quietly. #2074 made a missing GOVERNS link visible on stdout and in sentinel.md while #1990's hard-fail is still unbuilt. The three-lens follow-ups plan then split the next config-trust work into four independent tasks. This PR is Task C, issue #2065.

Strategic Shifts in This PR (if any)

No strategy change. pyproject.toml still declares dependencies=[], so this does not add jsonschema. The checker covers the small known key set (wrong type, missing required key, bad enum), not $ref or a full recursive schema. That matches the design's P1-1 scope note.

What This PR Shipped

• synlynk/config_schema.py exports validate(), CONFIG_SCHEMA, and POLICY_SCHEMA. validate() returns a list of human-readable errors. Config requires schema_version (int), budget (object with limit_usd and limit_requests), harness_billing (object), and workspace_id (string). identity_slug, local_fallback, and local_auto_threshold stay optional. Policy requires schema_version and repo_id, and checks capability_policy.mode against empirical and heuristic.
• synlynk doctor registers _hc_config_schema and _hc_policy_schema immediately after _hc_identity_slug. A missing file warns. A parse error or schema error fails and names the field. A matching file is ok.
• The live repo config (schema_version, budget, harness_billing, workspace_id) and policy (mode: empirical) both pass. A config that drops a required top-level field, or sets budget to a non-object, fails validation, so synlynk doctor no longer reports that file as OK.
• Tests: tests/test_config_schema.py (5) and tests/test_doctor_config_schema.py (2).

Plan: docs/superpowers/plans/2026-10-06-three-lens-followups-plan.md Task C. Design: docs/superpowers/specs/2026-10-06-three-lens-followups-design.md.

Brainstorm Visuals Used

None. This follows the committed design and plan.

What This Achieved on the Path to Autonomy

A bad config.json or policy.json now shows up in synlynk doctor before dispatch reads the wrong type or an illegal capability mode. The fleet still has no third-party schema dependency.

Strategic Note: The Goal at the End of This PR

The other three-lens follow-ups remain independent: sample-size-gated empirical routing (#2063), durable routing-fallback logs (#2064), and the cross-adapter conformance suite (#2062). #1990's GOVERNS hard-fail is still the next enforcement step for the warning #2074 shipped.