feat(core): add Stream Event Protocol v1 types (ADR 0001)

[sjr666666]

Summary

First step of #27 — the pure-type foundation for the Stream Event Protocol v1:

• packages/core/src/stream-events.ts: versioned envelope (protocolVersion, runId, messageId, monotonic sequence, timestamp, typed payload) plus 12 event types with a per-type payload mapping. Adds text_delta, tool_progress, citation_added, status, cancelled, error on top of the current 8-event AgentEvent.
• Re-exported from @finagent/core.
• docs/adr/0001-stream-event-protocol.md: context, decision, migration path (types first, then runtime / transport / renderer), and 3 open questions for reviewers.
• Unit tests asserting enum completeness and compile-time payload coverage.

No runtime/behavior change — AgentEvent and all existing consumers are untouched; the new protocol types coexist during migration.

Verification

• bun test packages/core/src/stream-events.test.ts → 4 passed
• tsc --noEmit for @finagent/core → exit 0
• Full-workspace typecheck on my local env fails on i18next module resolution (pre-existing under local bun 1.4 layout, unrelated to this diff; no files outside core/docs are touched)

Open questions

See ADR §Open questions: resume data source for v1, status.phase enum sufficiency, and keeping messageId merged with runId. Happy to adjust per review.

[helsome]

先暂缓单独合入。这个 PR 的 StreamEvent 判别联合问题已经由你在 #43 中确认并修复，而且 messageId === runId 与正在实现的 #34 conversation lineage 身份模型冲突。请以 #43 顶层 review 为准，先把 identity contract 定型，再合这组 stack。

[helsome]

更新审核结论：上一轮提到的 identity/reconnect 问题已经在顶层 #43 修掉，不再要求你回头改 #41。#41/#42 继续作为 stacked 中间层即可，不建议单独合；现在以已经 APPROVE 的 #43 为最终合入单元，剩余只需把顶层 stack rebase 到最新 main 并通过基础 CI。