Isolate concurrent Agent sessions

[hi-michael-li]

Why

Concurrent sessions could share provider, logger, skill, output, and lifecycle state. One session could leak data into or prematurely close another.

What

• Give every session a detached runtime with session-local tools, output/finish/cache state, skills, provider lifecycle, and logger lifecycle.
• Support overlapping sessions for exact built-in providers and loggers through private reconstruction rather than a public cloning protocol.
• Keep custom providers, custom loggers, and built-in subclasses usable sequentially; reject overlapping use of the same configured object before resource entry.
• Resolve the built-in ViewImage/code-backend dependency explicitly.
• Require SessionAgent.run() and run_tool() to execute only inside their own active context.
• Keep independent nested roots isolated and require delegated sub-agent tools to have an active parent session.
• Shield teardown from cancellation, preserve primary exceptions, and restore task-local state on every exit path.
• Keep signal handling main-thread/ref-count safe and reserve identical cache-enabled tasks through teardown.
• Preserve intentional Slack coordination objects while isolating each run's lifecycle.
• Require Rich 14.1 or newer for concurrent built-in live displays.

Deliberate limits

• Custom providers, custom loggers, and built-in subclasses are sequential-only when they share the same configured object; use separate instances for concurrency.
• A custom ViewImage/backend pair cannot borrow a different parent execution environment.
• Concurrent identical cache-enabled root runs are rejected; cache durability itself is handled by the cache PR.

Testing

• Focused session coverage: 27 passed.
• Full suite: 190 passed.
• Ruff, formatting, strict docs, package build, and diff checks pass. Type checking reports only the existing Slack optional-API warning.

[hi-michael-li]

Planned simplification

I’m going to narrow session isolation by removing the generic recursive provider/logger cloning framework:

• concurrent sessions remain supported for exact built-in providers and the built-in logger;
• custom providers, custom loggers, and built-in subclasses remain usable sequentially but overlapping use is rejected clearly;
• retain one task-local session context and explicit built-in dependency handling;
• reduce tests that only pin the generic cloning protocol.

This deliberately trades unsupported custom concurrency for a smaller, clearer ownership model.

[hi-michael-li]

Simplification complete

• Head: 61ae468
• Scope: 41 files (+4852/-689) → 12 files (+1987/-228)
• Rebuilt directly on main without a public provider/logger cloning protocol.
• Concurrent exact built-ins are supported; shared custom providers/loggers/subclasses are deliberately sequential-only and overlap-safe.
• Added explicit cancellation cleanup, session ownership, cache-task reservation, nested-root/subagent boundaries, and Rich 14.1 minimum support.
• Validation: 27 focused session tests; 190 full-suite tests; Ruff, formatting, strict docs, package build, type and diff checks pass (apart from the existing Slack type warning).