Docs: agent story README/a2a cross-links + Beta promotions (landed after #5409 merged)

[qqeasonchen]

Fixes the docs that missed the #5409 squash merge (they were pushed to the branch minutes after it was merged). Docs-only, no code. Single commit.

Lineage (verified by tree-hash audit)

Commit	What it is	State on develop
705163f25	#5408 squash merge — Agent extension points: LlmClient/ConversationMemory SPI, connector-backed agent tools and event triggers	✅ already merged; tree identical to the #5408 branch tip (69f5cc5d1) — nothing lost
2c738b7d7	#5409 squash merge — AgentTool SPI + module restructure (this PR's base)	✅ already merged; tree identical to the #5409 branch state at merge time (9be70b125) — nothing lost
this PR (6eae92b4b)	the only delta that missed develop: 4 doc files (+31/−7)	➕ complete: docs/agent-story-readme tree == the #5409 branch final state (abf98790a), so develop + this PR == the full reviewed branch content

What lands (4 files, +31/−7)

README.md / README.zh-CN.md

• New feature bullet: LLM streaming agent runtime — connector-as-tool, SPI agent plugins (eventmesh-agent-plugin/), event triggers with sink delivery
• New capability-table row: Agent tools & event triggers (Experimental) linking Agent extension points: LlmClient/ConversationMemory SPI, connector-backed agent tools, event triggers #5408/AgentTool SPI + module restructure: agent-plugin tree, agent -> agent-runtime, connector-api under connector-plugin (follow-up to #5408) #5409
• Capability promotions: Connector Runtime and A2A / Agent Gateway Experimental → Beta — the A2A promotion gate (Testcontainers E2E, [Architecture Review] A2A readiness: TaskExpirer, Meta-backed AgentCard, Runtime E2E, failure testing #5340) closed on 2026-09-08; wording updated from "Evaluate"/「评估」 to "Usable"/「可用」
• Docs-list entries for agent-tools (en/zh)

docs/feature/a2a.md

• New Related capabilities section: A2A (agent-to-agent) ↔ streaming agent runtime (agent-to-LLM), with the orchestrator → specialist-with-tools composition pattern

docs/index.md

• A2A entry cross-links its agent-tools companion page

All four files are byte-identical to the versions discussed on the #5409 branch (commits 13c5f500b + abf98790a).