feat: report agent runtime hours usage in entitlements (#27985)

Populate `FeatureAgentRuntimeHours.Actual` on every entitlements refresh
for licenses that grant the feature. A new
`GetTotalUsageHBAgentRuntimeV1` query sums `runtime_ms` over the
license's usage period, reading `usage_events` directly:
`hb_agent_runtime_v1` is exactly one row per hourly bucket
deployment-wide with `created_at` at the bucket start, enforced by the
unique partial index introduced in #27983.

The measurement reuses the shared `measureUsage` policy from #27984
through a new `AgentRuntimeMsFn` closure (usage publisher subject):
failures publish the stable
`LicenseAgentRuntimeUsageUnavailableErrorText` and log the cause. Usage
is floored to whole hours, matching the unit of the
`agent_runtime_hours_*` claims, and at most one warning is emitted per
refresh: reaching the allocation supersedes the advisory soft limit. The
dashboard renders the soft-limit advisory muted without a sales link and
treats the runtime usage-unavailable text as a diagnostic.

**Precise usage.** `Feature.ActualMs` (JSON `actual_ms`), set only for
`agent_runtime_hours`, carries the exact stored milliseconds backing the
floored `Actual` so clients can render fractional hours (e.g. `10.3`).
It has the same freshness as `Actual`; the whole-hour warning thresholds
are unchanged.

**Unlimited licenses.** A license minted with the unlimited (`-1`)
allocation decodes to an enabled feature with a nil `Limit` (#27984), so
the warning write-back now guards the allocation dereference: no
thresholds can exist for an unlimited license, so no runtime hours
warning is ever emitted, while `Actual` is still measured and published.
`Feature.Compare` is unchanged; for usage-period features the
issued-at/end dates decide first, so a metered feature outranks an
unlimited one only on an exact timestamp tie, an edge pinned by a
`TestFeatureComparison` case and documented on
`decodeAgentRuntimeHours`.

**Grandfathered premium licenses.** Premium licenses without
`agent_runtime_hours_*` claims are now granted the feature disabled with
a zero limit over the license term, identical to an explicit
`allocation: 0`: usage is measured and published for every Premium
deployment, and chatd's pooled admission (#27902) caps concurrent
agentic chats until a license with a positive allocation is added. The
default carries a fixed early `UsagePeriod.IssuedAt` (2026-08-01, the
same mechanism as the managed-agents default) so any license actually
carrying the claims outranks it in the `AddFeature` merge regardless of
the licenses' relative issue dates; the constant must stay earlier than
the earliest legitimately issued claim-bearing license. Zero allocations
(explicit or grandfathered) emit no deployment-wide warning banner:
those deployments are steered by the in-page upgrade CTA and the
concurrency cap. Enterprise licenses are unchanged.

Part 3 of a 3-PR stack splitting up #27796 (see there for review
history). Stack: #27983 → #27984 → this PR.

Closes CODAGT-852.

coderd/database/queries/usageevents.sql

@@ -117,3 +117,28 @@ WHERE
     -- Parentheses are necessary to avoid sqlc from generating an extra
     -- argument.
     AND day BETWEEN date_trunc('day', (@start_date::timestamptz) AT TIME ZONE 'UTC')::date AND date_trunc('day', (@end_date::timestamptz) AT TIME ZONE 'UTC')::date;
+
+-- name: GetTotalUsageHBAgentRuntimeV1 :one
+-- Gets the total Coder Agent runtime in milliseconds between two timestamps.
+-- The start bound is inclusive and the end bound is exclusive.
+--
+-- Unlike GetTotalUsageDCManagedAgentsV1 this reads usage_events directly
+-- rather than the usage_events_daily rollup: hb_agent_runtime_v1 is exactly
+-- one row per hourly bucket deployment-wide, with created_at at the bucket
+-- start, enforced by the unique partial index
+-- idx_usage_events_agent_runtime (which also keeps SUM from counting a
+-- bucket twice and serves this query). The result is bucket-granular: a
+-- bucket counts entirely against the period containing its start. See
+-- enterprise/coderd/usage/generator.go for what a bucket holds. If a
+-- usage_events retention policy ever lands, this must move to the daily
+-- rollup and accept day-granularity bounds.
+SELECT
+    -- The first cast is necessary since you can't sum strings, and the second
+    -- cast is necessary to make sqlc happy.
+    COALESCE(SUM((event_data->>'runtime_ms')::bigint), 0)::bigint AS total_runtime_ms
+FROM
+    usage_events
+WHERE
+    event_type = 'hb_agent_runtime_v1'
+    AND created_at >= @start_time::timestamptz
+    AND created_at < @end_time::timestamptz;