[FR-007] Orchestrator writes immutable manifest with hashes of all inputs/outputs

[explosivebit]

Trace: PRD prd-v0-1-smoke-evaluation-run · FR-007 · SPEC manifest schema · ADR-0002 (legacy run immutability)

Capability: After status → published, manifest.json content-addressed by run_hash, file mode chmod 0o444, R2 object-lock (v0.2+).

Acceptance:

• AC-2 from SPEC: published manifest cannot be mutated (write fails)
• All mandatory version pins present: methodology_version, inspect_ai_version, orchestrator_version, stack/model/task pins, seed_set, aggregates.counts_by_status

Implementation locus: apps/eval-core-py/src/orchestrator/manifest.py (state machine + atomic rename)