Add same-thread context clearing

[MateoCerquetella]

Human comments

What was wrong

BB had no same-thread provider-context boundary. Clearing context required a handoff, which split one piece of work across BB threads, while provider-session lookup always resumed the latest historical provider identity.

What changed

• Add a built-in /clear command that releases an idle runtime and records a visible Context cleared boundary without changing the BB thread, workspace, history, or sticky execution settings.
• Make provider-session lookup ignore identities before the latest clear boundary so the next prompt starts a fresh provider conversation.
• Serialize clears against prompt sends and reject active, starting, stopping, or interaction-blocked threads.
• Add the public API, SDK threads.clearContext, bb thread clear, guide, and bb-cli skill surfaces.
• Reuse the existing release-only thread.stop wire command; HOST_DAEMON_PROTOCOL_VERSION does not change.

How you verified

• pnpm exec turbo run typecheck --filter=@bb/domain --filter=@bb/db --filter=@bb/server --filter=@bb/sdk --filter=@bb/cli --filter=@bb/thread-view --filter=@bb/app --filter=@bb/server-contract --filter=@bb/templates
• pnpm exec turbo run lint --filter=@bb/domain --filter=@bb/db --filter=@bb/server --filter=@bb/sdk --filter=@bb/cli --filter=@bb/thread-view --filter=@bb/app --filter=@bb/server-contract --filter=@bb/templates
• Focused domain, DB, server, CLI, SDK, composer, thread-view, and template tests, including active-thread rejection, repeated clears, already-released runtimes, and send/clear races.

AGENT GENERATED