v0.6.4

Fixed first-message startup ordering

This patch fixes a regression shipped in session 0.6.3: a message sent immediately after creating a session could activate its turn before the initial agent/model operations ran. The initial agent switch then marked that turn as a failed handoff, leaving the session without an assistant reply.

Initial agent and model operations now complete before user-message admission. SESSION_OPENED remains the first upstream event. Existing agent/model handlers, persistence, switch notices, and agent-default model selection are preserved. Initialization errors stop startup rather than admitting input against incomplete configuration. The init function remains in the ordinary command queue.

No new configuration, SDK methods, or database migration is required. Existing steering, message-receipt-after-persistence, Stop, and real agent-switch behavior remain covered by the test suite.

Verification: all 325 session tests pass on runtime 0.3.44a, and GitHub test/lint checks pass. Removing the startup drain reproduces both regressions (zero assistant replies), with the other 323 tests still passing; restoring it makes the full suite pass.

The separate cached-context checkpoint fix in #47 is not included in this release.