feat(agent): stable instance ID and per-instance state directory

[ccf-lisa]

Part 4/11 of the split of #95 (agent remote configuration) into a stack. Review it as the diff against its base branch.

The heartbeat used a new random UUID every run, so every restart looked
like a new agent. Each instance now has a stable ID (R31) kept in a state
directory:

• internal/agentstate: Store opens the state directory (0700), probes it
  and keeps working in memory when it is not writable; InstanceID reads,
  creates or replaces a corrupt instance-id file atomically.
• The directory defaults to .compliance-framework/state/<sha256(abs config
  path)[:16]>; --state-dir / CCF_STATE_DIR pin it, --instance-id /
  CCF_INSTANCE_ID pin the ID (not persisted). The agent logs where state
  lives at startup, since moving the config file creates a new instance (R52).
• AgentRunner takes the ID (WithInstanceID) and the heartbeat uses it.

The heartbeat test lives in cmd/reconciler_test.go, the file the reconciler PR (#106) fills in, so the final layout matches.

Size: 443 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.