Flaky under load: run note tests, session temp-dir races, workspace and provider timing

[santoshkumarradha]

These tests fail intermittently under load on santos/dev2. They also fail on a clean trunk, so they are not caused by the changes they were seen beside. Each was rerun alone to confirm.

Test	Package	Seen	Rate on clean trunk
TestNotesBeyondTheBoundWaitForTheNextBoundary, TestANoteIsHandedToAWorkerOnceAndTheScreenStillReadsIt, TestANoteDoesNotChangeWhatATaskWasAskedFor	internal/run	"stopped at its step cap after 9 steps"	about half the runs at load 27 to 35 on 20 cores (3 of 5 alone)
TestTwoWorkersLiveAtOnceEachGetOnlyItsOwnNote	internal/run	note routing	4 of 20
TestARunsRowNamesTheStoreTaskThePlanReadAnswersUnder	internal/session	temp directory cleanup race	3 of 20
TestANewDirectionArrivesWithTheClaimUnspent	internal/session		1 of 10
TestEightyConcurrentAddsFromSeparateHandlesAllLand	internal/workspace	only under load; passes alone
TestAWireBelowItsPaceWithNothingToSpendSaysSoAndIsSpent	internal/provider	failed in 2 full runs; passed alone 3 of 3

The internal/run note tests are the costly ones: they turn a full make check red about half the time, so a green check on trunk is partly luck.

Suggested order: the run note tests first (a step cap reached under a slow scheduler reads as a real failure), then the two temp-directory races.

[AbirAbbas]

Rechecked #1410 at 008363c98 on 2026-09-24. The run worker's step handshake removes the stopped at its step cap after 9 steps cause from the note tests: 3,000/3,000 passes under -cpu 1,2,8 -count=1000 at low load. Under load 10–19, 3 of 600 runs instead failed at step 1 with the run's wall stopped the worker. Their 10-second test-harness wall elapsed while the test's own store write was in SQLite fsync, before the first tool-end handshake. A separate capped-step finish incorrectly reported as the cap remains and is being fixed on #1410.

[santoshkumarradha]

More tests that fail only under load, seen on santos/dev2 during full make check runs on 2026-09-24. Each one's package code is identical to trunk.

Test	Package	Seen
TestPlandbCliConcurrentWritersLandEveryTask	internal/plandb	failed once in an overloaded run
TestTheLosingArmsRowSaysItIsExhaustAndNotAFailure	internal/provider	race test; passed 30 of 30 alone on the branch and on trunk
TestS3ItComesBack	internal/lane	1 in 20 alone on a branch, 20 of 20 on trunk
TestAJudgedWideTaskWhoseFirstPartsAreRefusedStillGainsTheDivideVerb	internal/session	failed once in a suite run; passed 10 of 10 alone
TestAgreementDoesNotBuyASecondCompletion	internal/session	hit its 60 s wait right after another suite released the lock; passed 10 of 10 alone