Add autonomous finish contract

[sethkarten]

Summary

• add an autonomous finish contract that prevents stopping with only "I think it works"
• require evidence before autonomous mode stops: git patch, passing test/check evidence, blocker artifact, or justified no-op
• feed a host-side continuation prompt when the finish contract is not satisfied
• keep the feature scoped to autonomous mode and leave normal sessions unchanged

Testing

• npm --prefix packages/coding-agent test -- suite/agent-session-autonomous.test.ts
• ./node_modules/.bin/tsgo --noEmit
• npm run check
• stacked autonomous suite on feature/autonomous-quality-gates: npm --prefix packages/coding-agent test -- suite/agent-session-autonomous.test.ts args.test.ts

Note

Add autonomous finish contract to require evidence before stopping autonomous agent sessions

• Introduces a finishContract concept in autonomous.ts: when enabled (default), the agent checks for concrete finish evidence before accepting a stop decision.
• Evidence is detected via hasAutonomousFinishEvidence, which checks for pending git worktree changes (git status --porcelain) or text patterns indicating passing tests, a blocker artifact, or a justified no-op.
• If evidence is missing, shouldAutonomouslyContinue returns a new 'finish_contract' decision reason, and nextAutonomousContinuation injects a specific prompt (DEFAULT_AUTONOMOUS_FINISH_PROMPT) demanding concrete evidence.
• AgentSession._getContinuationMessages now passes { cwd } to nextAutonomousContinuation so git status checks use the correct working directory.
• Behavioral Change: autonomous sessions enabled by default will no longer stop unless the agent produces a git patch, passes tests, references a blocker, or explicitly justifies a no-op.

^{Macroscope summarized 9f03440.}

[sethkarten]

Closing as superseded by #222. The finish-contract behavior is now implemented there as host-side terminal-evidence checks instead of a separate PR.