feat(agent): prepare-then-cancel config file reloads

[ccf-lisa]

Part 6/11 of the split of #95 (agent remote configuration) into a stack. Review it as the diff against its base branch.

Replace the reload loop (cancel on every file event, reload, panic on an
invalid file) with a reconciler goroutine that is the single writer of the
configuration state (R32):

• A trigger loads the file on a fresh viper and prepares a complete
  candidate (runtime conversion + Prefetch) BEFORE cancelling the running
  configuration. An invalid file or a download failure keeps the running
  configuration; only an unusable file at startup exits 1, and a startup
  download failure still sends the startup-failure agent evidence.
• File events are debounced; a candidate equal to the running one (a
  comment-only edit) is recorded without a restart.
• A candidate that prepares but fails to run falls back to the previous
  configuration once and enters a failed backoff (1m doubling to 10m), so
  it is not re-applied on every tick.
• The file watcher only signals; loading always happens in the reconciler.

remote_config.poll_interval (default 60s) paces the retry tick here; the reports and overlays that also use it land in #108 and #110.

Size: 867 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.