refactor(core): simplify session context epochs - #33378

Summary

• remove Context Epoch replacement, agent ownership, and revision state
• treat agent and model selection as provider-turn scoped while preserving chronological ContextUpdated messages
• rebuild the cached system baseline directly after completed compaction
• rely on the process-local Session execution lane instead of defending unsupported overlapping same-Session drains

Verification

• bun run --cwd packages/core test test/session-runner.test.ts test/session-projector.test.ts test/session-runner-recorded.test.ts test/session-compaction.test.ts test/system-context/index.test.ts test/database-migration.test.ts (122 passed)
• bun run --cwd packages/core typecheck
• bun run --cwd packages/core migration --check
• pre-push bun turbo typecheck (23 tasks passed)
• full Core suite before final simplification: 1,033 passed; one unrelated watcher test consistently expects change but receives add for .git/HEAD