buzz-acp: bound ACP session + provider-resource lifecycle (no eviction today)

[salman1993]

Background

Split out of the thread-scoped ACP sessions PR (#6732) review (reviewer P1).
Not introduced by that PR — it is a pre-existing lifecycle-hardening concern
that thread scoping makes more visible because it multiplies the number of
live sessions (one per thread instead of one per channel).

Problem

Sessions are retained indefinitely:

• The pool's per-scope maps in buzz-acp (sessions, turn_counts,
  core_sections, canvas_sections, deliveries, session_owners) have no
  time- or size-based eviction. They only shrink on explicit triggers
  (!rotate, channel/membership removal, agent exit/panic). Under thread
  policy a busy channel can accumulate one entry set per thread and never
  release them.
• buzz-agent sessions can retain their McpRegistry resources. A finite
  BUZZ_AGENT_MAX_SESSIONS exists but defaults to unlimited.
• ACP has no obvious per-session close contract, so deleting only the
  Buzz-side session IDs would not reliably release provider-side resources
  (subprocesses, MCP servers, model context).

Why it isn't a clean PR-local fix

A partial eviction (dropping Buzz-side IDs) would leak provider resources
and give a false sense of boundedness. This needs a designed lifecycle:

• an idle-TTL and/or LRU cap on pool scope maps, keyed by SessionScope,
• a real per-session close/teardown contract that releases provider resources
  (ACP session close, McpRegistry teardown),
• a sensible finite default for BUZZ_AGENT_MAX_SESSIONS,
• metrics/telemetry for live-session count and evictions.

Acceptance (proposal)

• Bounded live sessions per agent under sustained multi-thread load.
• Evicting a session provably releases provider-side resources.
• Defaults are safe for the default channel policy (no behavior change) and
  bound growth under thread policy.

References

• PR feat(buzz-acp): give each channel thread its own agent session #6732 (thread-scoped ACP sessions)
• crates/buzz-acp/src/pool.rs (scope maps, invalidate_*)
• buzz-agent McpRegistry, BUZZ_AGENT_MAX_SESSIONS