**Stack**: #28359 → #28360 → **#28361 (this PR)** → #28362
During local tool execution, buffer episodes now record when each
tool-call occurrence actually starts and finishes, via
`RecordToolStart`/`RecordToolCompletion`/`ToolCompletions` on the
message part buffer. Occurrences are keyed by unresolved-call position,
so gaps left by rejected calls and duplicate tool-call IDs stay
distinct: `partitionAmbiguousToolCalls` additionally returns the allowed
calls' input positions, and generation's `chatloop.ToolBillingRecorder`
implementation translates dispatch-order stamps back through them.
Mechanism only: nothing reads the stamps yet. The final PR in the stack
uses them to bill interrupted tool batches on a dedicated usage record
committed alongside their synthesized cancellation rows.
Third of a four-PR stack splitting #28211. Stacked on #28360. Refs
CODAGT-928
(https://linear.app/codercom/issue/CODAGT-928/track-local-tool-execution-for-agent-runtime).
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Co-authored-by: Hugo Dutka <hugo@coder.com>
