fix(agent-core-v2): surface SessionStart hook stdout in context and transcript

[haiyu614]

Related Issue

Fixes #2873

Problem

SessionStart hook commands execute, but the session-scope external hooks service discards the returned HookResult[] — triggerSessionStart() never publishes a hook.result event and never appends the hook stdout to the conversation context. Users see nothing despite the documented hook contract ("stdout content (if any) may be appended to context"), and the model never receives SessionStart output either.

Verified on 2.1.1 (Windows): a SessionStart hook with echo hello-from-sessionstart-hook writes its side effects but the main agent's wire.jsonl contains no hook.result event and no context entry for the stdout.

Fix

After the session-scope trigger completes, SessionExternalHooksService now surfaces successful hook results through the main agent:

• extracts non-empty messages from results with action !== 'block' (reusing the same extraction rules as UserPromptSubmit: skip timed out / errored / non-zero exits, prefer message, fall back to stdout),
• appends each message to the main agent's context as an assistant message with origin: { kind: 'hook_result', event: 'SessionStart' } and the same <hook_result hook_event="..."> wrapper used by other hook results, so the model sees the output and session replay renders it as a transcript entry,
• dispatches a hook.result agent event per message, so attached clients (TUI / print / SDK) render it exactly like UserPromptSubmit hook results.

If the main agent does not exist yet (session created without a main agent binding), the results are dropped as before — no behavior change in that edge case. SessionEnd / heartbeat / subagent triggers remain fire-and-forget.

hookResultMessage() (previously the private userPromptHookMessage) is exported from internal/userPrompt.ts so both scopes share the same extraction rules.

Tests

• New integration test: surfaces SessionStart hook stdout on the main agent as a hook result and context entry — fires the SessionStart lifecycle event with a real runner, asserts the context append (role/origin/wrapped text) and the dispatched hook.result event, and asserts SessionEnd output is still not surfaced.
• Existing session-hook tests updated with an IAgentLifecycleService stub (the service now injects it).
• pnpm --filter @moonshot-ai/agent-core-v2 test: no new failures vs. clean baseline on Windows (the pre-existing failures are identical before/after; new test passes).
• Manual end-to-end check: kimi -p with a SessionStart hook now records the <hook_result hook_event="SessionStart"> context message in the main agent's wire.jsonl.

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Latest commit: b0a0f89

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@moonshot-ai/kimi-code	Patch

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