fix(outcome): preserve colliding decision receipts

[solomonneas]

Summary

• Give outcome decision receipts collision-resistant filenames even when decisions share a second or artifact IDs normalize to the same slug.
• Publish fully written JSON through an exclusive hard link, so concurrent writers cannot replace an existing receipt and readers never observe partial JSON.
• Keep loading legacy second-resolution receipt filenames through the existing *.json discovery path.

Root cause

Decision receipt paths used a second-resolution timestamp plus a lossy artifact slug. The mutable JSON helper then published with os.replace, so a later decision selecting the same path silently replaced the earlier receipt.

Verification

• RED receipt 20260728-025815-work-verify-492587: 5 collision and exclusivity tests failed against the previous production behavior.
• GREEN receipt 20260728-030103-work-verify-35a8c4: deterministic slug collisions, atomic publication, 8 concurrent fixed-path writers, concurrent decision-writer retries, retry exhaustion, and legacy loading passed.
• Focused suite receipt 20260728-025848-work-verify-cf2722: 119 outcome and scorecard tests passed.
• Full gate receipt 20260728-030128-work-verify-e1e0e0: 4,662 passed, 3 skipped, 82.93% coverage, with one shared editable-install failure in test_verify_reused_receipt_stamps_harness_session_from_outer_env_prefix.
• Clean origin/main baseline receipt 20260728-031031-work-verify-7a5022: the same isolated test failed with the same No module named brigade stderr under the shared virtualenv override. The test was not changed or skipped. CI's clean install is the deciding gate.
• Independent review after the atomic-publication fix found no Critical, Important, or Minor findings.

Closes #564

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