Fix race condition in concurrent task execution

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Fixes #1

Changes

workspace/task/runner.go

• Replaced all direct field access (task.State, task.Err, task.History, task.StartedAt, task.FinishedAt) with thread-safe setter methods (SetState, SetErr, SetStartedAt, SetFinishedAt)
• Eliminated manual task.mu.Lock()/Unlock() blocks in favor of encapsulated methods
• State transitions are now atomic through the existing SetState method which handles locking internally

workspace/task/task.go

• Added SetStartedAt and SetFinishedAt methods for thread-safe write access to time fields

workspace/task/runner_test.go

• Fixed TestRunner_Run to use GetState() instead of direct t1.State field access (race condition in test itself)
• Added TestRunner_Run_StressWithCancel — 100 concurrent tasks with mid-execution cancellation, concurrent metadata reads/writes, and state checks. Verifies go test -race passes cleanly.

Acceptance Criteria Met

• All data races eliminated: no direct concurrent field access
• go test -race ./... passes with zero data race warnings
• Task state transitions (Pending -> Running -> Completed/Failed) are atomic via SetState with mutex
• Concurrent writes to shared maps/structs synchronized via existing mutex-protected getters/setters
• Context cancellation handled gracefully with proper state transition to Failed

/claim #1

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