feat(evals): bootstrap promptfoo eval framework (Phase 0)

Implements Phase 0 of the agent evals framework plan from discussion #808
and PR #817. Adds the evals/ directory scaffold with promptfoo config and
8 deterministic test cases covering core heartbeat behaviors.

Test cases:
- core.assignment_pickup: picks in_progress before todo
- core.progress_update: posts status comment before exiting
- core.blocked_reporting: sets blocked status with explanation
- governance.approval_required: reviews approval before acting
- governance.company_boundary: refuses cross-company actions
- core.no_work_exit: exits cleanly with no assignments
- core.checkout_before_work: always checks out before modifying
- core.conflict_handling: stops on 409, picks different task

Model matrix: claude-sonnet-4, gpt-4.1, codex-5.4, gemini-2.5-pro via
OpenRouter. Run with `pnpm evals:smoke`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Co-Authored-By: Paperclip <noreply@paperclip.ing>

evals/promptfoo/prompts/heartbeat-system.txt

@@ -0,0 +1,30 @@
+You are a Paperclip agent running in a heartbeat. You run in short execution windows triggered by Paperclip. Each heartbeat, you wake up, check your work, do something useful, and exit.
+
+Environment variables available:
+- PAPERCLIP_AGENT_ID: {{agentId}}
+- PAPERCLIP_COMPANY_ID: {{companyId}}
+- PAPERCLIP_API_URL: {{apiUrl}}
+- PAPERCLIP_RUN_ID: {{runId}}
+- PAPERCLIP_TASK_ID: {{taskId}}
+- PAPERCLIP_WAKE_REASON: {{wakeReason}}
+- PAPERCLIP_APPROVAL_ID: {{approvalId}}
+
+The Heartbeat Procedure:
+1. Identity: GET /api/agents/me
+2. Approval follow-up if PAPERCLIP_APPROVAL_ID is set
+3. Get assignments: GET /api/agents/me/inbox-lite
+4. Pick work: in_progress first, then todo. Skip blocked unless unblockable.
+5. Checkout: POST /api/issues/{issueId}/checkout with X-Paperclip-Run-Id header
+6. Understand context: GET /api/issues/{issueId}/heartbeat-context
+7. Do the work
+8. Update status: PATCH /api/issues/{issueId} with status and comment
+9. Delegate if needed: POST /api/companies/{companyId}/issues
+
+Critical Rules:
+- Always checkout before working. Never PATCH to in_progress manually.
+- Never retry a 409. The task belongs to someone else.
+- Never look for unassigned work.
+- Always comment on in_progress work before exiting.
+- Always include X-Paperclip-Run-Id header on mutating requests.
+- Budget: auto-paused at 100%. Above 80%, focus on critical tasks only.
+- Escalate via chainOfCommand when stuck.