fix: nudge startup prompts for promptless role agents

[pxjx22]

Problem

When Gas Town spawns witness or refinery sessions using a promptless runtime (e.g. Codex, where `prompt_mode: none`), the startup patrol prompt was silently dropped. The sessions received the fallback `gt prime && gt mail check --inject` commands but never received the actual patrol instruction, leaving them idling while appearing healthy.

The effect is operationally serious: ready merge queue entries sit unclaimed indefinitely and role agents appear running while doing nothing. Polecats already had the correct fallback path implemented — witness and refinery did not.

Fix

• Added `GetStartupPromptFallback` helper to `internal/runtime` to detect when a startup prompt must be delivered via nudge for promptless runtimes
• Wired `internal/witness/manager.go` and `internal/refinery/manager.go` to send the patrol prompt after startup fallback completes
• Added unit coverage in `internal/runtime/runtime_test.go`

Verification

• Confirmed live witness and refinery sessions now receive the patrol prompt after startup under Codex
• Confirmed stuck merge queue entries were subsequently claimed and merged