fix: gate fork affordances on completed replies

[wibus-wee]

Summary

• Streaming assistant replies no longer render the Fork button or its "Fork session" affordance; they expose context copying through a direct Copy action instead.
• AssistantForkButton and the pending-fork loading state now require message.finished === true; a pending fork no longer keeps the action group force-visible on an unfinished turn.
• Lands the previously unmerged fix (be09f427, authored 2026-09-11) rebased onto current main: the streaming context-copy test is ported to the createConversationViewFromHistory harness from feat: read long conversations by window with shared history writes #376, the ai-gui AGENTS.md rule is added within the 8 KiB gate, and the Agent Note ships its zh counterpart.
• Spec conversation-context-copy-and-text-attachments updated (still draft): a streaming reply must not show a Fork affordance or Fork loading state.

Test plan

• NODE_ENV=test pnpm vitest run tests/session-fork-destination-menu.test.tsx tests/chat-virtual-rows-identity.test.ts tests/assistant-turn-action-inset.test.ts — 23 tests pass (streaming case asserts no fork button, direct copy action works; finished case keeps fork menu)
• pnpm --filter @lody/components typecheck — clean
• pnpm run docs check — 0 errors
• pnpm lint:fast / lint:i18n — clean
• pnpm format — no changes needed

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