Governance config: one-time bootstrap load + two-level lazy TTL (stop fetching GET /v1/governance/{agent} on every run start) #114

Problem

tokenops_run() calls governance_config_for(service) on every run start / agent hop unless an explicit governor= is passed (src/tokenops/control/run.py:224-226).

• Embedded mode routes through governance_cache.py (process cache keyed (store_path, agent), invalidated on Store governance writes) — effectively free after the first call.
• Remote / HTTP mode routes client.governance_config_for -> HttpStore.governance_config_for -> GET /v1/governance/{agent} with no cache. Every run start is a blocking round trip to the plane for config that only changes on Admin edits.

Under the HTTP hard-dependency (see #55, #60), and given TokenOps runs per turn, this adds a plane round trip to every turn purely for near-static config.

Desired behavior

Client-side governance-config cache that:

1. Loads once on application bootstrap (or lazily on first use) and is held in an in-memory store keyed by (base_url, agent).
2. Two-level TTL, both evaluated lazily on request arrival (when a run hits the agent) — not on a background timer:
   • Soft TTL: past it, the cached value is still served for this run, and a background refresh is kicked off to rebuild from a fresh GET /v1/governance/{agent}. No request blocks.
   • Hard TTL: past it, the cache entry is considered stale-unsafe — the next request blocks on a synchronous rebuild before building the Governor.
3. On a successful fetch (foreground or background), the entry is replaced and both TTL clocks reset.
4. Explicit invalidation still works (Admin edit -> clear_governance_config_cache, or a plane-push/webhook later).
5. Fail-open on background refresh error (keep serving the last good value until hard TTL); fail-closed on hard-TTL refresh error (surface it — do not run ungoverned, cf. Fail-closed when a run has no governed dispatch, instead of silently running ungoverned #109).

Notes / scope

• Generalize governance_cache.py so it wraps the HTTP path, not just the embedded Store path.
• TTLs configurable via env (e.g. TOKENOPS_GOVERNANCE_SOFT_TTL_S, TOKENOPS_GOVERNANCE_HARD_TTL_S) with sane defaults (soft ~60s, hard ~600s — tune).
• Background refresh: a single-flight worker per (base_url, agent) so concurrent runs don't stampede the plane.
• Re-key the existing cache from (store_path, agent) to (base_url, agent) in remote mode.
• Keep the deep-copy-on-read semantics (build_governor mutates its input dict).

Acceptance sketch

• Steady-state: 0 GET /v1/governance/{agent} calls on the run hot path; refreshes happen in the background after soft TTL.
• A config change in Admin is picked up within soft TTL (background) or forced at hard TTL.
• Plane outage shorter than hard TTL is invisible to runs.

Related: #55, #60, #109.