fix(server): reserve the dispatcher role for multi-agent work

[lukemaj]

What: Agents are now told to start a dispatcher only when work needs several agents coordinated; a single bounded change goes to one worker.
Why: The tool instructions said every one-PR job should go to a dispatcher, so a small UI change got a coordinator on the dispatcher model list (GPT-6.1 Sol) instead of a worker on the configured worker lane.
So what: Merging needs the user's approval; afterwards new agent sessions pick up the wording.

Changes the guidance in apps/server/src/mcp/toolInstructions.ts and the spawn_thread description in apps/server/src/mcp/toolkits/threads/tools.ts, plus the test that pins the instruction text.

Proof: pnpm exec vp test run apps/server/src/mcp/toolInstructions.test.ts gave 7 passed.

Elon record

• Requirements and who asked: the user asked that dispatchers be used only for multi-agent coordination on bigger work, not quick changes.
• Deleted: the "job that ends in one PR" trigger for dispatchers.
• Bottleneck: the instruction text agents read at session start; no code path chooses roles.
• Checked myself: grepped apps and packages; the two strings above (and their test) are the only places that route one-PR jobs to a dispatcher.

🤖 Generated with Claude Code

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.5 KiB	13.5 KiB	−13 B (−0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	0 B (0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.4 KiB	6.4 KiB	−13 B (−0.2%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.2 KiB	56.2 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	9	9	0 (0.0%)	21	✅
Claude	Total thread wire	13.5 KiB	13.5 KiB	+9 B (+0.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	−1 B (−0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.4 KiB	6.4 KiB	+10 B (+0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.0 KiB	57.0 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: 7dd561e · PR result: 42e04d6 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 114.0 KiB
• Claude decoded thread snapshot: 114.7 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.