feat: confidence-gated push-context channel — `kb.volunteer_context`

[plind-junior]

What you're trying to do

gbrain's docs/guides/push-context.md documents three push channels — ambient reflex, volunteer_context op, gbrain watch — through which the brain volunteers confidence-gated pages into the conversation window. Today vouch is purely pull: the agent asks, vouch answers. Adding a push channel lets vouch surface "you have an approved claim about this; consider citing it" mid-session.

Suggested shape

• New kb.volunteer_context MCP op (server → client streaming notification).
• Server-side trigger: kb.session_start opens a watch on hot-memory + salience deltas; when a claim's relevance score for the active session exceeds a config threshold (default 0.85), push a notification with {claim_id, relevance, why}.
• Throttled to one push per 30 seconds; rate-limited per session.

Acceptance

• A session that asks about jwt sees a volunteered claim within 5 seconds if a high-relevance approved claim exists.
• The push respects scope visibility (per chore(deps): bump actions/checkout from 4 to 6 #2).

Out of scope

• Two-way conversational volunteering — push only.
• Volunteering proposals (not just durable claims).