Serialize run.json writes and seed turnsUsed/mcpServers on resume

[TheGreatAxios]

Summary

• Chains saveState writes per sessionId so a straggler progress snapshot can no longer race a terminal finalize write and resurrect status: "running" on a finished session
• Resolves a resumed session's turnsUsed and mcpServers once at the resume boundary (resolveResumeSeed), so createRunSink, connectedMcpServers, and the immediate post-resume saveState all read a trusted seed instead of resetting to zero/empty or each repeating its own default

Verification

• bun run typecheck passes
• bun run build passes
• bun test: 4319 pass, 0 fail
• New tests: src/session/state.test.ts (straggler-vs-terminal race, cross-session isolation), src/session/run-sink.test.ts (seeded turn count), src/tui/resume-seed.test.ts (resolveResumeSeed with non-empty mcpServers and non-zero turnsUsed fixtures, plus the pre-mcpServers-field record case)
• Known gap, accepted deliberately: resolveResumeSeed and createRunSink's seeding are unit-tested, but the three call sites inside runTUI that consume resumeSeed (the createRunSink call, the connectedMcpServers initializer, the post-resume saveState) are not exercised by any test — runTUI has no test harness anywhere in the repo (it mounts a real TUI host/reactor), and tests/e2e fixture-repo runs are listed as planned, not yet built, in AGENTS.md. Building that harness for this fix is disproportionate: each call site is a single-line read of an already-tested value, so a regression there is a one-line diff that's easy to catch in review. Revisit if runTUI gets a real integration harness for other reasons.

Closes CL-5567
Closes CL-5566

[linear-code]

CL-5567

CL-5566