fix(core): restore default model headers

[rekram1-node]

Summary

• add shared session-owned model request headers matching V1 affinity, session, parent, and user-agent behavior
• preserve V2 project, session, and client correlation headers
• apply the defaults to runner, title, and compaction model calls without adding plugin hooks

Validation

• bun test in packages/core (1285 passed, 6 skipped)
• bun test test/session-runner.test.ts test/session-title.test.ts test/session-compaction.test.ts in packages/core (134 passed)
• bun typecheck in packages/core
• normal pre-push hook workspace typecheck (32 packages)