fix(dreamer): advance last_run_at from run start, not completion

[TreedsSlop]

last_run_at is the cutoff the "changed since" gates compare against the timestamps of the input they govern (maintain-docs: compartments since the last run). advanceAfterRun wrote the run's COMPLETION time, so a message or compartment that arrived during the run was older than the new cutoff and was silently skipped on the next slot.

Record the run's start instead: input that lands mid-run is newer than the cutoff and re-triggers the gate next slot (at-least-once).

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Summary by cubic

Fixes last_run_at advancement in the dreamer scheduler so input that arrives mid-run re-triggers the "changed since" gates next slot instead of being silently skipped.

• Records the run's start time as the cutoff rather than its completion time.
• Failed/skipped runs still never advance the cutoff; nextDueAt still advances from completion.

^{Written for commit 3bf683c. Summary will update on new commits.}

Confidence Score: 5/5

The PR appears safe to merge, with no outstanding correctness, security, or repository-rule issues identified.

Summary

Updates Dreamer scheduling bookkeeping so successful tasks record their start time as last_run_at, preserving at-least-once processing for inputs that arrive while a task is running.

• Keeps failed and skipped runs from advancing the successful-run cutoff.
• Continues calculating next_due_at from completion time.
• Adds scheduler coverage for the start-versus-completion distinction and updates executor expectations.

Diagram

sequenceDiagram
    participant S as Scheduler
    participant E as Executor
    participant DB as Schedule State
    S->>S: Record startedAt
    S->>E: Execute due task
    Note over E: New input may arrive during execution
    E-->>S: Completed
    S->>DB: "last_run_at = startedAt"
    S->>DB: "next_due_at = schedule from finishedAt"
    Note over DB: Mid-run input remains newer than cutoff

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