refactor(runtime): route turn producers through dispatcher

[Neonforge98]

Problem

Agent turn producers currently own overlapping copies of optimistic status, turn-generation reservation, transport dispatch, activity timestamps, and terminal waiting. Composer, queue, side chat, mode-switch resume, plan execution, Work Item retry, and address-comments can therefore drift in idempotency and finality behavior.

Solution

• Route interactive and headless Agent sends through the canonical TurnDispatchService introduced by the stacked dispatcher PR.
• Preserve UI-specific transcript writes at the producer boundary while deleting duplicated transport/FSM bookkeeping.
• Make address-comments wait for the exact dispatched intent outcome instead of maintaining a second terminal watcher.
• Keep durable WorkItemRun retry as the primary path and route only the legacy local-session fallback through canonical dispatch.
• Leave Team Chat and its human-message plane unchanged; audience routing remains owned by the separate routing PR.

Potential risks

• This PR is intentionally stacked on feat(runtime): add canonical turn dispatch boundary #941 and should be reviewed or merged after fix(runtime): make turn acceptance retry-safe #940 and feat(runtime): add canonical turn dispatch boundary #941.
• Headless paths now inherit durable receipt reconciliation and exact-generation finality from the dispatcher, so previously silent failed/cancelled address-comment turns are surfaced as failures.
• No cloud API, conversation-plane schema, Team Chat transport, or deployment workflow changes are included.

Validation

• pnpm typecheck
• ESLint and Prettier on all 16 changed files
• 8 focused Vitest files, 40 tests passed
• Madge scan from the migrated entry points: 984 files, no circular dependency
• Repository commit hooks passed

[Neonforge98]

Superseded by the final two-PR design: #844 removes the Work Item-specific remote continuation bridge, and stacked #939 keeps the reusable audience routing plus sender-local provider-native continuation. This dispatcher/store/handoff stack is intentionally retired.