feat(usage): attribute every auxiliary LLM call in canonical token usage

Goal

Audit every model/provider invocation outside the normal agent-run stream and ensure its input/output usage is recorded in the canonical durable Session.tokenUsage ledger under a purpose-specific usage kind and server-selected provider/model attribution.

The session-title generator is the first migrated auxiliary caller (session_title). Other LLM-backed operations may currently discard usage or fold it into the main run incorrectly.

Acceptance criteria

• Inventory all composition/server/agent auxiliary LLM call sites, including their ownership, lifetime, session association, and current usage handling.
• For every session-associated call, record usage in a closed, purpose-specific canonical usage kind; preserve Session.Usage, EvResult.Usage, and MaxRunTokens as main-run-only unless an explicitly documented policy says otherwise.
• For calls that cannot be tied to a session, document the intentional accounting boundary rather than fabricating attribution.
• Preserve provider neutrality: do not widen port.LLMRequest for accounting or provider-private controls.
• Add offline coverage for every migrated call and update living architecture/implementation documentation.