Keep pinned runs outside completed retention quota

[dennisonbertram]

Closes #1048

Summary

• count only zero-subscriber persisted terminal candidates toward completed-run retention pressure
• preserve subscriber-pinned terminal runs as protected exceptions
• preserve persistent run records, terminal persistence gating, configuration, and wire behavior

Root cause

Subscriber-pinned terminal states consumed MaxCompletedRetention despite being ineligible for pruning. Under a limit of one, that allowed a just-completed run to be removed before its StartRun caller could subscribe.

Test-first evidence

Hosted race CI exposed the existing TestRunner_PruneKeepsCompletedRunWithActiveSubscriber regression as collect extra run 2 events: run not found before the production fix.

Exact-head verification

Rebased retention-only head d8e7c261fc5680c10d599be080844d0837587b98 onto production main f3fb4d8e911210d53cb43e8e3163b24bda70cca1.

• focused retention test normal -count=100
• focused retention test under race detector -count=100
• complete internal/harness package normal/race
• foreground non-TTY ./scripts/test-regression.sh: PASS, 85.6% total coverage, zero uncovered functions
• regression now proves both that the subscriber-pinned run survives and that the unpinned retention quota remains bounded

Rollout / rollback

No migration or flag. Revert this commit if subscriber cancellation fails to reclaim pinned state or the number of unpinned drainable terminal states exceeds the configured limit.

Related: #958, #1000, #1047, #1045, #1043, #1041, #1033.

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[dennisonbertram]

Exact tested head: 5b77220. @codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

Reviewed commit: 5b772204b0

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[dennisonbertram]

@codex review

Exact-head review requested for rebased retention-only commit 118d1a2 on merged main 8906149. Focused/adjacent/complete harness normal and race checks plus the foreground non-TTY full regression gate passed; net scope remains six #1048 files.

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

Reviewed commit: 118d1a230b

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[dennisonbertram]

Exact-head review requested for 82e1e6c. This is rebased onto production main f3fb4d8. Focused retention test passed 100x normally and 50x under race; complete internal/harness normal/race and foreground ./scripts/test-regression.sh passed with 85.7% coverage and zero uncovered functions. @codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Another round soon, please!

Reviewed commit: 82e1e6c629

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