feat(server): add project-scoped thread supervision

[lukemaj]

Summary

• Add list_threads with child and same-project scopes, settled filtering, and thread metadata.
• Extend read_thread and message_thread with scoped access and sender attribution for peer threads.
• Preserve the fork bookkeeping and add focused scope tests.

Closes #15

Proof

• pnpm install --frozen-lockfile
• pnpm exec vp run -r typecheck
• pnpm exec vp lint
• pnpm exec vp fmt --check
• pnpm exec vp test run apps/server/src/mcp/toolkits/threads
• scripts/fork-check.sh
• Isolated live MCP check on 127.0.0.1:3799 with T3CODE_HOME=/tmp/t15/live-home: two independent same-project threads were listed, read, and messaged; the target stored the attributed sender message.

The isolated provider turn reported the expected account/model compatibility error for codex/gpt-5; the thread toolkit operations and message persistence succeeded.

Model and harness: GPT-5.6, Codex in T3 Code.

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	13.5 KiB	—	15.1 KiB	✅
Codex	Thread snapshot wire	—	7.1 KiB	—	7.3 KiB	✅
Codex	Live turn WebSocket wire	—	6.5 KiB	—	7.8 KiB	✅
Codex	Live turn WebSocket decoded	—	56.3 KiB	—	66.4 KiB	✅
Codex	Live turn messages	—	10	—	21	✅
Claude	Total thread wire	—	13.5 KiB	—	15.1 KiB	✅
Claude	Thread snapshot wire	—	7.1 KiB	—	7.3 KiB	✅
Claude	Live turn WebSocket wire	—	6.4 KiB	—	7.8 KiB	✅
Claude	Live turn WebSocket decoded	—	57.0 KiB	—	66.4 KiB	✅
Claude	Live turn messages	—	9	—	21	✅

Baseline: unavailable · PR result: df043e0 · 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: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

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