feat(events): add SESSION_START and SESSION_END lifecycle events

[galuszkm]

Description

External consumers of the event stream had no reliable way to know the topology of a session before agent activity started, and no typed signal that a session had cleanly ended.

This PR adds two session-level lifecycle events that bracket every invocation, giving consumers a deterministic start and end marker with rich metadata attached.

• SESSION_START — first event on the queue, emitted synchronously before any agent runs. Carries a SessionManifest in data.
• SESSION_END — last typed event before the end-of-stream sentinel. Carries {"session_id": <str|null>} so consumers can finalise UI state deterministically without relying on the sentinel.
• New strands_compose.manifest module with pure builder functions build_manifest() and build_session_manager_descriptor().
• New SessionManifest Pydantic v2 models in types.py: AgentDescriptor, OrchestrationDescriptor, EntryDescriptor, ModelDescriptor, NodeRef, EdgeRef, SessionManagerDescriptor.
• EventQueue.emit_session_start(manifest) — synchronous, guarded against double-emission.
• EventQueue.flush() resets both SESSION_START and SESSION_END guards in addition to draining the queue.
• wire_event_queue() builds the manifest and emits SESSION_START before returning.
• AnsiRenderer updated to handle SESSION_START and SESSION_END

Related Issues

N/A

Type of Change

• New feature

YAML / API Impact

No YAML schema changes. All new Python API is additive:

• New keyword-only parameters on make_event_queue() (entry_name, session_id) default to None — existing callers are unaffected.
• wire_event_queue() behaviour is unchanged for single-turn use. Callers that reuse the queue across turns (e.g. multi-turn servers) must call flush() then emit_session_start() explicitly after each flush to re-emit SESSION_START per turn.
• New public symbols: SessionManifest, AgentDescriptor, OrchestrationDescriptor, EntryDescriptor, ModelDescriptor, NodeRef, EdgeRef, FileProviderDescriptor, S3ProviderDescriptor, AgentCoreProviderDescriptor, CustomProviderDescriptor.

Fully backwards-compatible.

Testing

How have you tested the change?

• I ran uv run just check (lint + type check)
• I ran uv run just test for overall testing
• I added or updated tests that prove my fix is effective or my feature works
• I verified existing examples in examples/ still work

Checklist

• I have read the CONTRIBUTING document
• I have updated the documentation accordingly
• My changes generate no new warnings
• Any dependent changes have been merged and published

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