feat(agent): AgentRunner primitives for prepare-then-cancel reloads

[ccf-lisa]

Part 5/11 of the split of #95 (agent remote configuration) into a stack. Review it as the diff against its base branch.

The building blocks the reconciler in the next PR drives:

• Prefetch downloads every plugin and policy source of a candidate config
  and resolves plugin protocols without touching the running config's
  locations, so a download failure never tears a working agent down (R32).
  ReportStartupFailure keeps main's startup-failure behaviour (plugins
  marked failed, startup-failure agent evidence) for a failed prefetch.
• Plugin protocol versions read from OCI annotations are cached per source
  and survive reloads, so a registry outage cannot turn a v2 plugin into v1.
• runDaemon returns setup errors instead of exiting. On a reload, in-flight
  plugin runs drain for up to 5 minutes (R33); a SIGINT/SIGTERM during the
  drain still exits within 30s. Each cron job runs with the config, client
  and logger of its own setup.
• UpdateConfig reuses the SDK client when the api block is unchanged, so
  its token cache survives reloads.
• Plugins get the host environment minus CCF_API_AUTH_* (R26).
• make test runs with -race.

The agent still uses the old reload loop here; #106 replaces it with the reconciler that drives Prefetch. Tests live in cmd/reconciler_test.go (filled in by #106).

Size: 489 lines changed (go.mod/go.sum excluded). Builds, vets, is gofmt-clean and passes go test -race ./... on its own; the top of the stack is byte-identical to #95 merged with main.

Stack

1. test: pin agent configuration hashes and evidence seeds #101 test: pin agent configuration hashes and evidence seeds
2. feat(runner): policy source from the _policy_path label; extra evidence props #102 feat(runner): policy source from the _policy_path label; extra evidence props
3. refactor(config): adopt api/pkg/agentconfig as the declared config #103 refactor(config): adopt api/pkg/agentconfig as the declared config
4. feat(agent): stable instance ID and per-instance state directory #104 feat(agent): stable instance ID and per-instance state directory
5. 👉 feat(agent): AgentRunner primitives for prepare-then-cancel reloads #105 feat(agent): AgentRunner primitives for prepare-then-cancel reloads
6. feat(agent): prepare-then-cancel config file reloads #106 feat(agent): prepare-then-cancel config file reloads
7. feat(pluginlib): read the agent library version a plugin binary was built with #107 feat(pluginlib): read the agent library version a plugin binary was built with
8. feat(agent): report the configuration to the API (remote_config off/report) #108 feat(agent): report the configuration to the API (remote_config off/report)
9. feat(agentstate): persisted cache for the remote configuration overlay #109 feat(agentstate): persisted cache for the remote configuration overlay
10. feat(agent): pull and apply remote configuration overlays (apply_safe/apply_all) #110 feat(agent): pull and apply remote configuration overlays (apply_safe/apply_all)
11. feat(config): ${env:NAME} placeholders in plugin config; ADR 0003 #111 feat(config): ${env:NAME} placeholders in plugin config; ADR 0003

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[gusfcarvalho]

ccf-review: APPROVE

no findings.

Stack (gh stack 112): #101 → #102 → #103 → #104 → #105 → #106 → #107 → #108 → #109 → #110 → #111

[ccf-lisa]

PR approved. Marking ready for e2e.