post: report the mount lifecycle to the vm-agent at post time

[piob-io]

Summary

The post step sends one structured report to the vm-agent so the host's per-mount lifecycle record can include what only the guest knows. The host record never depends on it: the report is additive, best-effort, and dropped silently if the agent is old or unreachable.

src/mount-report.ts (new) — same transport as the checkout action's /internal sender:

POST http://$BLACKSMITH_AGENT_ADDR:$BLACKSMITH_METRICS_HTTP_PORT/internal
{ metric_type: "stickydisk_mount_report", value: 1, vm_id, attributes: {}, payload: MountReport }

interface MountReport {
  expose_id, sticky_disk_key: string;
  setup_outcome: "mounted" | "setup_fallback";
  skip_reason: "" | "commit_false" | "early_deny" | "if_missing_existing" | "on_change_unchanged"
             | "prior_step_failure" | "step_check_error" | "setup_error" | "not_mounted" | "post_error";
  was_formatted: boolean;
  format_ms, mount_ms, unmount_ms: number;
}

• skipped (returns false) when either env var is missing; resolves true only on a 2xx response, any other status is a failed send like a network error; every failure swallowed at core.debug, never surfaced to the job.
• MOUNT_REPORT_TIMEOUT_MS = 1500. The agent is on the runner's local network, so a healthy round trip is milliseconds, and the post step sits on the customer's critical path: a stalled agent costs at most 1.5s (down from the initial 3s).
• skip_reason is "" when the commit was requested.

src/post.ts — the report is built up front from state, each existing skip branch tags its skip_reason, and sendMountReport runs in a finally so exactly one report leaves per post run, including the early "not mounted" returns and the catch-all error path.

src/main.ts:

• STICKYDISK_EXPOSE_ID is saved right after GetStickyDisk returns instead of after mount, so a guest-side format/mount failure still reports against the disk the host exposed (and the post step now sends shouldCommit=false for it rather than skipping the RPC).
• mkfs/resize2fs and mount+chown durations are timed and saved as STICKYDISK_FORMAT_MS / STICKYDISK_MOUNT_MS.

Receiver side is in FastActions/fa#5523 (joins the report onto the buffered row by expose_id; sticky-disk-key fallback for failed exposes that have no expose id).

Tests (src/__tests__/mount-report.test.ts) cover env parsing, the envelope, a real local HTTP server receiving the POST, a local server answering 500, missing endpoint, refused connection, and a silent server hitting the timeout.

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Requested by: @piob-io

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Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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[ajwerner]

Reviewed the follow-up changes; failed-phase timings are preserved in the source and rebuilt action bundles.