The ai-seats cron registration fix now ships independently in #27508 so
it is not coupled to this feature's Tallyman release gate. The
usage-publisher create permission stays here because the generator's
heartbeat inserts also require it; both PRs carry the identical hunk.

…time (#27312)

closes CODAGT-839
closes CODAGT-843
closes CODAGT-773

## Summary

Adds a new heartbeat usage event type, `hb_agent_runtime_v1`, measuring
the total agent-loop runtime of Coder Agents (chats) per UTC hour, plus
a reconciler that generates one event per hour with self-healing
backfill over a trailing 7-day window. Events flow to Tallyman through
the existing publisher unchanged. This measures the new Coder Agents
(the `chats` tables), not the deprecated Tasks counted by
`dc_managed_agents_v1`.

Independent of #27508, which fixes the dead ai-seats cron registration.
Both PRs carry the identical `usage_event` create permission hunk for
the usage-publisher subject (this feature's generator and the ai-seats
cron each need it for heartbeat inserts), so they can land in either
order and the overlap merges cleanly.

> [!WARNING]
> **Do not include this in a release until Tallyman accepts
`hb_agent_runtime_v1`.** The publisher marks permanently rejected events
as done-forever, and the generator then sees those buckets as complete
locally, so their usage would be silently and permanently lost.

## Details

Each event's payload is `{"runtime_ms": N}`: the sum of
`chat_messages.runtime_ms` for messages created in the hour bucket `[H,
H+1)`, across all chats (sub-agents, API-created, archived, and
soft-deleted messages included). Events use deterministic IDs
(`hb_agent_runtime_v1:<bucket start>`) with `created_at` set to the
bucket start, so concurrent replicas race safely via `ON CONFLICT (id)
DO NOTHING` without locking, and daily rollups attribute backfilled
hours to the correct day. Idle hours produce zero-valued events. A
bucket becomes eligible 5 minutes after it closes; hours missing for
longer than the 7-day window are forfeited, which can only undercount.

Note that this makes `usage_events.created_at` explicitly the *event
occurrence time* rather than the row insertion time; the two only
diverge for backfilled events. It already behaved as the occurrence
timestamp (it drives the daily rollup day and is shipped to
Tallyman/Metronome as the event timestamp), and the migration now
documents this with a `COMMENT ON COLUMN`, which also surfaces as a Go
doc comment on `UsageEvent.CreatedAt`.

The new `usage.Generator` runs unconditionally in enterprise builds; the
`publish_usage_data` license flag continues to gate egress only, so
air-gapped deployments still fill their local ledger. The
`aggregate_usage_event()` trigger sums `runtime_ms` per day into
`usage_events_daily` (unlike `hb_ai_seats_v1`, which takes the daily
max).

`InsertHeartbeatUsageEvent` now takes an explicit `createdAt` so
generators can backfill historical buckets; the cron passes
`clock.Now()` to preserve its existing behavior.

## Tallyman follow-up

<details>
<summary>Prompt for the Tallyman-repo agent</summary>

> **Task**: Add support for the new Coder usage event type
`hb_agent_runtime_v1` so Tallyman accepts, validates, and forwards it to
Metronome.
>
> **Background**: coder/coder PR (this PR) adds hourly heartbeat events
measuring Coder Agent runtime. Events arrive via the existing
`/api/v1/events/ingest` endpoint with: `event_type:
"hb_agent_runtime_v1"`, `event_data: {"runtime_ms": <int64 >= 0>}`,
deterministic `id` of the form `hb_agent_runtime_v1:2026-07-15_14:00:00`
(UTC hour bucket start), and `created_at` set to the bucket start (may
be up to ~8 days in the past due to backfill; within Metronome's 34-day
dedup window). Zero-value events are normal (idle hours).
>
> **Work**:
> 1. Update Tallyman's vendored/imported `coderd/usage/usagetypes` (or
equivalent) to the coder/coder commit that adds
`UsageEventTypeHBAgentRuntimeV1` and `HBAgentRuntime`.
> 2. Ensure ingestion validation accepts the type (`Valid()` switches)
and rejects negative `runtime_ms`.
> 3. Ensure Metronome forwarding maps the event with transaction ID
derived from the event `id` as for existing types, passing `runtime_ms`
through as the property for a SUM-aggregated billable metric ("Coder
Agent Hours" = `SUM(runtime_ms) / 3,600,000`).
> 4. Do NOT permanently reject unknown-but-well-formed future `hb_*`
types if avoidable; at minimum confirm current behavior for unknown
types (temporary vs permanent rejection) and report it.
> 5. Tests: ingest accept/validate, dedup by ID, Metronome payload
mapping.
>
> **Constraint**: this must be deployed to tallyman-prod **before** any
coder/coder release containing the event generator; coderd treats
permanent rejections as terminal per event.

</details>