[Gastown] Wire Detailed Polecat System Prompt

[jrf0110]

Parent: #204 | Priority: P1 — Low effort, high impact on agent quality

Problem

A detailed polecat system prompt exists at cloudflare-gastown/src/prompts/polecat-system.prompt.ts (tested with 6 passing tests in polecat-system.prompt.test.ts). It includes:

• GUPP principle instructions
• Tool documentation for all 8 gt_* tools
• Workflow guidance (check mail → work on bead → checkpoint → done)
• Git workflow instructions

But the Rig DO's startAgentInContainer() calls systemPromptForRole('polecat') which returns a much simpler, generic prompt from a static method. The detailed prompt with GUPP instructions and tool documentation is never used.

Fix

Replace the systemPromptForRole('polecat') call in startAgentInContainer() with buildPolecatSystemPrompt() from prompts/polecat-system.prompt.ts. Pass the rig config (git URL, default branch, agent identity) so the prompt can be customized per-agent.

This should also be done for the other roles as they get real system prompts (refinery, witness, mayor).

Acceptance Criteria

• startAgentInContainer() uses buildPolecatSystemPrompt() for polecat role
• Prompt includes agent identity, rig context, GUPP instructions, tool docs
• Existing polecat system prompt tests continue passing
• Verify prompt is actually received by kilo serve session (log or test)

[jrf0110]

Closed via PR #353. buildPolecatSystemPrompt() and buildMayorSystemPrompt() wired into Rig DO systemPromptForRole(). Also fixed premature polecat exit (session.idle no longer treated as terminal).