session: per-(channel, speaker) session management with conversation continuity

[matedev01]

Goal: per-(channel, speaker) sessions with conversation continuity, bounded for the Jetson.

Sub-steps

• Session key = (channel, speaker/chat-id).
• Wire session → conversation history (per session).
• Lifecycle: create / idle-expire, with a bounded cap so memory stays in budget.

M2 · Workstream A.

[matedev01]

Progress: the session key (channel, speaker/chat-id) landed in #611 — the HTTP chat + stream paths carry resolved speaker identity into the session/memory context, with a channel-invariance conformance test.

Remaining:

• Wire session → conversation history (per session).
• Lifecycle: create / idle-expire, with a bounded cap so memory stays in the Jetson budget.

Keeping open for those two.