v0.7.8 — deterministic completion policy

Turns completion accept/reject from control-flow booleans into an explicit,
inspectable CompletionPolicy over harness-built EvidenceContracts. Three modes
via FORGE_COMPLETION_POLICY: audit (record, never block), balanced (default —
reject a failed check once, then permit the historical second-claim escape as a
named single_bounce_override), and strict (require passing verification for every
changed workspace). Each decision is logged as completion_policy with its
evidence and attempt number, and the final decision is attached to accepted
assistant records. Balanced mode is outcome-compatible with prior behavior while
making the former invisible escape hatch explicit. README documents the modes.