feat(agentstate): persisted cache for the remote configuration overlay

[ccf-lisa]

Part 9/11 of the split of #95 (agent remote configuration) into a stack. Review it as the diff against its base branch.

/remote-config.json (0600, written atomically) keeps the last fetched
and applied overlay with its raw ETag, and the rejected-revision memory, so
the next PR can keep running the last good overlay while the API is
unreachable and re-report a rejection after a restart (R7).

• The cache is bound to api.url and api.auth.client_id: a mismatch
  discards it.
• A checksum detects corruption; LoadCache returns ErrCacheCorrupt and an
  empty cache so the agent continues without it.
• IfNoneMatch returns the raw ETag the API sent, never one built from a
  revision number.

Size: 203 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.