[Feature Request]: support herdr natively like pi-herdr (agent-state integration)

Summary

Add native Herdr support so jcode is recognized and tracked as a first-class agent by Herdr, the same way pi, claude, and codex are, instead of Herdr relying on fragile screen-scraping of the TUI.

Problem

Herdr (herdr.dev) is a terminal multiplexer/runtime for coding agents. It identifies agents and tracks their session state through per-agent integration files. Today jcode is completely absent from that mechanism:

• herdr integration status lists pi, claude, codex, omp, opencode, etc. - no jcode entry.
• herdr agent list never shows a jcode agent.
• Herdr therefore falls back to screen-scraping jcode's TUI (spinner frames, status labels), which is fragile and version-dependent (see #574).

This mirrors #638, which is currently triage: needs-decision. This issue narrows that request into a concrete, actionable mechanism so it can be decided and shipped. Commenter ppronobis on #638 put it plainly: "now it is not recognized as an agent."

What "native like pi-herdr" means concretely

Every well-supported agent has an integration file Herdr invokes over its unix socket (pane.report_agent_session) to report session identity/path/status:

• claude: /root/.claude/hooks/herdr-agent-state.sh (v7)
• codex: /root/.codex/herdr-agent-state.sh (v6)
• pi: /root/.pi/agent/extensions/herdr-agent-state.ts
• opencode: /root/.config/opencode/plugins/herdr-agent-state.js

These are installed/managed by herdr integration install <agent>. Native support means jcode provides (or Herdr can install) a herdr-agent-state hook/extension that jcode invokes at session start/completion to report its session to Herdr, so:

1. Herdr recognizes the pane's agent as jcode (correct identity, not screen-scraped).
2. agent_status reflects real jcode state (idle/working/blocked/done) rather than spinner heuristics.
3. Session transcripts (agent_session) are linked so Herdr can resume/inspect jcode sessions.

Requested scope

• Add a Herdr agent-state integration for jcode (shell hook or native extension) covering the pane.report_agent_session protocol, mirroring claude/codex.
• Have jcode invoke it on session start and on status transitions (working -> idle/done/blocked).
• Ensure herdr integration status and herdr agent list report a jcode agent.
• Keep it as a fallback-safe integration (fail closed to exit 0 on missing env/socket), matching existing hooks.

Related

• #638 - general "Jcode X Herdr colab" request (parent of this one)
• #574 - emit OSC 9 progress for reliable status detection (complementary; OSC progress + agent-state hook together give Herdr both identity and status)
• #405 - client terminal env propagation for tmux/zellij; herdr env vars are already snapshotted there

Start with Herdr's pane.report_agent_session protocol and compare the listed claude, codex, and pi integration files, then trace where jcode handles session start and status transitions. Done means Herdr recognizes jcode, reports idle/working/blocked/done state, links sessions, and safely exits when the required environment or socket is missing.