docs: cover MeshJob event injection shipped since v2.1.0 (concepts, env vars, SDK refs, release notes)

[dhyansraj]

Summary

The six commits shipped since v2.1.0 (#1041, #1043, #1045, #1047, #1049, #1051) deliver MeshJob event injection across Python / TypeScript / Java but documentation is still at the pre-ship state. This issue covers the docs-only half (concepts, env vars, release notes, SDK references). A follow-up issue will cover the runnable example.

Gaps

Stale claims

• docs/concepts/stateful-agents.md:396-405 still says event injection is "Coming soon" / "on the roadmap as issue MeshJob: event injection into running jobs — recv_event + send_event + Stream subscription + trace propagation #1032". Contradicts main.

Missing concept coverage

• docs/concepts/jobs.md documents the v2.0 baseline only (submit / wait / progress / retry_on / cancel). No mention of:
  • Point-to-point event injection (recv_event / send_event / post_event and their TS/Java siblings)
  • Stream subscription mode (subscribe_events / subscribeEvents)
  • Synthetic cancel event semantics (200ms grace window)
  • Typed errors (JobNotFoundError / JobTerminalError and language siblings)
  • LRU JobProxy cache
• docs/concepts/index.md has no event-injection breadcrumb.

Missing env-var documentation

• docs/environment-variables.md — missing MCP_MESH_JOBPROXY_CACHE_MAX (default 256, the LRU cache cap) and MCP_MESH_CANCEL_EVENT_GRACE_MS (default 200ms, the synthetic-event grace window after cancel).
• meshctl man environment-variables — same gap. Should mirror the docs page.

Missing SDK reference entries

• Python (docs/python/...), TypeScript (docs/typescript/...), Java (docs/java/...) reference pages lack entries for the new SDK methods.

Missing release notes

• RELEASE_NOTES.md has the v2.1.0 entry only. No v2.2.0 stub describing what shipped since.

Scope

• Update docs/concepts/jobs.md with an Event injection section covering recv/send/post and a Stream subscription section covering subscribe. Cross-runtime examples (Python/TS/Java) per the project doc convention.
• Replace the "Coming soon" block in docs/concepts/stateful-agents.md with an accurate cross-link to the new sections.
• Add the two env vars to docs/environment-variables.md and the corresponding meshctl man environment-variables content.
• Add SDK reference entries in docs/python/, docs/typescript/, docs/java/.
• Add a v2.2.0 stub at the top of RELEASE_NOTES.md.

Out of scope (separate issue)

• Runnable example agent demonstrating event injection (separate issue / PR, will reference back to this one).

Acceptance

• All references to "Coming soon" for event injection removed.
• mkdocs serve renders the new sections cleanly (grid cards, code fences, cross-links).
• meshctl man environment-variables output includes the two new env vars.