feat: add session.turn.completed bus event for plugin hooks

[xbjpku]

Issue for this PR

Closes #23503

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a session.turn.completed bus event that fires at the end of each assistant turn — after all tool calls finish, before compaction runs.

Why: I'm building a filesystem sandbox plugin (ScarletWitch) that needs to know when a turn ends so it can query COW state and show a review dialog. The existing plugin hooks don't cover this timing:

• tool.execute.after fires per tool call, not per turn — showing a dialog mid-turn would interrupt the LLM
• No existing bus event signals turn completion
• chat.message fires on message receipt, not after tool execution finishes

The change is +9 lines across 2 files:

• session/session.ts (+6): Define Session.Event.TurnCompleted with sessionID payload
• session/prompt.ts (+3): yield* bus.publish(Session.Event.TurnCompleted, { sessionID }) after the runLoop exits

Zero breaking changes — the event is additive. Existing code is unaffected.

How did you verify your code works?

• Tested with the ScarletWitch-opencode-plugin: server plugin receives the event via the event hook, TUI plugin receives it via api.event.on(), COW review dialog shows correctly after each turn
• Verified the event fires at the right time (after all tool calls, before compaction) by checking supervisor logs

Screenshots / recordings

N/A — this is a backend event, no UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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[github-actions]

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