**Stack**: #28359 → #28360 → #28361 → **#28362 (this PR)**
## Summary
Interrupted local tool batches now bill the partial window they actually
executed, recorded on the same dedicated usage record completed batches
use. Completes the runtime billing work in this stack: completed batches
bill via #28360, interrupted batches via this PR.
Last of a four-PR stack splitting #28211. Stacked on #28361. Closes
CODAGT-928
(https://linear.app/codercom/issue/CODAGT-928/track-local-tool-execution-for-agent-runtime).
## Problem
Interrupting a batch previously billed only the model window; work the
tools already performed was lost.
## Fix
Just before closing the buffer episode, the interrupt task reads the
live tool stamps (`ToolCompletions`) alongside `ModelInvokedAt`. The
interruption commit appends the shared `batchUsageMessage` record (same
shape as #28360: model-only tool-role row with a
`billed_ms`/`billed_calls` audit payload) billing the union of the
started, billable calls' partial execution intervals: completed calls
end at their recorded completion, running calls at the interrupt
instant. Synthesized cancellation rows stay free of batch-level runtime.
Unstarted calls, rejected calls, and sub-agent orchestration tools bill
nothing and append no record. Batches with no live attempt (crash
recovery, state promotion) bill nothing, consistent with generation
losing in-flight runtime on a crash.
Usage-record billing reuses the same interrupt instant as the model
window instead of taking a second clock reading inside the transaction,
so database delay cannot inflate a running call's window.
An earlier revision carried a retry-stable episode snapshot so an
interrupt retry after buffer eviction (a >15s stall) would not lose
billing; it was dropped as not worth the complexity
(https://github.com/coder/coder/pull/28211#discussion_r3812778418). Such
a retry now bills the batch at zero, matching the existing
crash-recovery precedent.
Known gaps left out of scope: retry backoff and errored-attempt time
stay unbilled (no persistence home for error turns), and the advisor
tool's nested model call now bills as tool time by design.
Jaayden Halko
511dcc0293