feat(runtime): compose steerable provider sessions

[drewstone]

Summary

• Opt-in provider steering delegates to the existing createSteerableSandboxSession; the low-level one-shot provider executor is unchanged.
• Turn aborts call the active AgentSession.cancel(), and provider-specific stream/cancel failures become the existing interrupt signal so a forced steer can re-plan.
• Exact AgentProfile identity, provider-owned backend defaults, provider options, normalized events, usage fields, live progress, and trace data survive the adapter boundary.
• Providers must declare live streaming and session continuation before this mode creates an environment.

No new agent loop or provider-specific coordinator is introduced.

Proof

• Focused provider/session checks: 119/119
• Full tests: 1,863/1,863 active; 6 skipped
• Lint: 502 files
• TypeScript and examples: pass
• Package build, exports, publint, attw, edge checks: pass
• Generated API reference and freshness check: pass
• Independent contract re-audit: APPROVE, 0 unresolved findings

Retained risk

The package has no configured third-party environment provider, so the live network-provider check remains an integration responsibility; the PR proves the public Interface 0.39 contract with exact provider events and session controls.

@tangle-network/agent-provider-cli-bridge@0.2.33 advertises continuation but does not expose environment.session(), so this mode rejects it before creation. @tangle-network/agent-provider-tangle@0.4.5 exposes the required session controls; adding them to the CLI-bridge provider is a separate upstream change.