feat(buzz-acp): isolate sessions by conversation thread

[bbamnesia]

Summary

Give each Buzz conversation thread its own ACP session while preserving channel-level behavior for unthreaded messages and DMs.

• derive a typed conversation scope from the channel and canonical NIP-10 root
• scope forum roots by their own event ID so forum posts and comments share one session
• key queueing, in-flight work, cancellation, rotation, and session state by conversation scope
• allow sibling threads in one channel to execute concurrently without batching or steering each other
• keep session ownership sticky to one agent while history exists, including when that agent is busy
• apply desktop model changes across every scoped session in a channel without leaking models to unrelated channels
• resolve metadata before subscribing to dynamically joined channels so the first event cannot be routed under the wrong scope
• keep DMs channel-scoped

Behavior

• replies in one thread reuse one ACP session
• separate threads in the same channel use separate ACP sessions
• separate forum posts use separate ACP sessions
• top-level unthreaded messages retain channel-level continuity
• !cancel, !rotate, queue deduplication, and retry state target the current conversation scope
• channel removal still drains all scopes under that channel

Tests

• cargo test -p buzz-acp — 605 passed
• cargo fmt -p buzz-acp -- --check — passed
• cargo clippy -p buzz-acp --all-targets -- -D warnings — passed
• cargo check -p sprig — passed
• workspace Rust fmt/clippy — passed
• desktop TypeScript/Biome/policy checks — passed
• desktop Tauri fmt/clippy — passed
• web checks — passed

just ci reaches the workspace unit suite but current main has an unrelated deterministic buzz-db failure in replica_fence::tests::fence_starts_closed_and_opens_on_advance: the assertion compares a microsecond-truncated stored timestamp with its original nanosecond value. The same isolated test fails without involving any files changed here.