Move abort operation from ISession to ITurn

[mikea]

Motivation

• Cancellation should be turn-scoped rather than session-scoped so callers can only abort the active turn (ITurn) instead of the whole session (ISession).
• Keep the public API spec and implementation in sync by moving the abort capability to the turn object and updating dependent code and docs.

Description

• Added abort(): Promise<void> to ITurn and removed abort() from ISession in the public API (packages/api/src/index.ts and specs/api.md).
• Updated AgentSessionDO to construct TurnImpl with an abort delegate (#abortCurrentTurn) and removed the session-level forwarder for abort() in SessionTarget (packages/core/src/agent-session-do.ts).
• Exposed TurnImpl.abort() which invokes the DO's abort delegate to cancel the active turn stream.
• Updated gateway and CLI usage: web UI now resolves session.getCurrentTurn() and calls turn?.abort(); Telegram /abort and gateway examples in specs/gateway.md and specs/telegram_gateway.md were updated to call getCurrentTurn()?.abort().
• Updated test mocks and test code to match the new API shape (web mock ITurn includes abort(), core mocks removed ISession.abort() no-op, and in-DO tests now call getCurrentTurn()?.abort() where appropriate).

Testing

• Ran unit tests for core with pnpm -C packages/core test, all tests passed (314 tests passed).
• Ran web gateway tests with pnpm -C gateways/web test, all tests passed (3 tests passed).

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Codex Task