refactor(omnio): use Omnio paths for worker artifacts

[ppazosp]

Worker summaries and transcript paths exposed to Toolbox should belong to Omnio regardless of the agent harness. New worker artifacts now use /tmp/.omnio-session/cache/delegation/ through the existing file synchronization mechanism. The harness keeps its own private profile storage; no new Toolbox API or transport is introduced.

This follow-up to #112 changes only the worker artifact namespace. General Hermes execution scratch paths and non-Sprites backends retain their current behavior. Existing Toolbox copies are not moved or removed.

Backward compatibility

Backward compatible. New Hermes uses the same authenticated /files operations against existing Toolbox versions, which accept the new path. Old Hermes works with new Toolbox assets, including the retained .hermes-session generation-transfer exclusion. No Omnia client, database, event, or capability contract changes. Existing copied files keep their original paths for their current lifetime; these caches remain ephemeral across generation transfer. Older transfer helpers may copy the new cache until the companion Omnia PR #4693 is present, without affecting file access. No coordinated rollout is required.

Verification

All eight Python CI slices passed at d0e737e5d4, along with the other code checks: run 35328573361.

Canonical scripts/run_tests.sh, with file retries disabled: 143 passed, 0 failed across delegation artifact delivery, live transcripts, credential mapping, file synchronization, and Sprites environment suites. Ruff and git diff --check pass. The artifact suite uses real local HTTP requests and separate harness/Toolbox directories; the summary and log assertions independently require the Omnio path. This namespace-only follow-up has not been rerun against a live Sprite; #112 retains the prior real-Sprite evidence for the unchanged transport.

#	Behaviour (it name)	Category	Precondition / input	Observable outcome asserted	Type	Status
1	summary path can be read completely from Toolbox	Happy	spilled worker summary	Omnio path returns every byte through HTTP	Integration	✅ test_delegation_artifact_delivery.py › test_summary_path_can_be_read_completely_from_toolbox
2	live log refreshes before each parent read	Happy	appended worker transcript	Omnio log path exposes the appended text	Integration	✅ test_delegation_artifact_delivery.py › test_live_log_refreshes_before_each_parent_read
3	large artifact uses raw upload without truncation	Edge	artifact larger than JSON write cap	remote bytes match complete input	Integration	✅ test_delegation_artifact_delivery.py › test_large_artifact_uses_raw_upload_without_truncation
4	invalid artifact destination is rejected	Error	path escapes artifact root	error and no file request	Unit	✅ test_delegation_artifact_delivery.py › test_invalid_artifact_destination_is_rejected
5	failed transfer is reported and retried	Error	write failure then recovery	failed attempt is reported; later remote file is complete	Unit	✅ test_delegation_artifact_delivery.py › test_failed_transfer_is_reported_and_retried