refactor(agent-core-v2): align xstate machine naming with command/fact/streaming conventions

[sailist]

Related Issue

N/A — internal refactor, no user-facing change.

Problem

The XState machines in agent-core-v2 had drifted from the naming conventions we adopted (XState official guidance + messaging convention: commands imperative, events past tense): noun/past-participle command events (turn.notifications, input.reminder), mixed segment casing (llm.message-id kebab, turn.spawnTools/turn.remindersConsumed camel), preposition state names (before/after in the tool machine), and no visible distinction for streaming data events (llm.delta/headers/usage/finish/message-id).

What changed

Pure renames, no behavior change; wire contracts (klient / kap-server / bus event names) are untouched.

• Event naming rule documented: events are classified by what the receiver does with them — command (imperative verb), fact (past participle), or data stream (noun). Added docs/en/event-name.md and docs/zh/event-name.md under packages/agent-core-v2/docs/.
• Command events fixed: turn.notifications→turn.notify, input.reminder→input.remind, turn.spawnTools→turn.spawn_tools.
• Fact events aligned: turn.remindersConsumed→turn.reminders_consumed, turn.start→turn.started (matches the bus event name), agent.error→agent.failed, tool.async→tool.detached.
• Data-stream events namespaced: llm.delta→llm.streaming.part, llm.headers→llm.streaming.headers, llm.usage→llm.streaming.usage, llm.finish→llm.streaming.finish, llm.message-id→llm.streaming.message_id. The five identical streaming handlers in the agent and session machines each collapse into one 'llm.streaming.*' wildcard declaration.
• Tool machine states: before/after→preparing/finishing (invoke actors renamed to preparingActor/finishingActor accordingly).
• Synced the acp-server test stub (LlmRequestEvent is a public subpath export) and the shorthand event lists in docs/{en,zh}/llm.md.

providerCatalog's naming deviations are deliberately excluded: PR #3606 is rewiring that file, its rename will follow after that lands.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue (external PRs: the issue must have a maintainer's /approve).
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

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