Add prime-agent (pa) as a first-class herdr agent kind

[bangedorrunt]

Summary

Add prime-agent as a first-class herdr agent kind (pa / prime-agent). It already ships a built-in herdr socket reporter, and herdr already accepts its lifecycle reports as hook authority — but the kind is not in the Agent enum, so herdr agent start --kind pa fails (unsupported_agent_kind) and agent prompt/agent send-keys/agent explain reject every prime-agent seat (effective_known_agent() is None).

Evidence (verified 2026-08-15, herdr 0.8.0-preview.2026-08-04-d78e3d3b5126)

• prime-agent v0.7.2 ships packages/coding-agent/src/core/extensions/builtin/herdr-agent-state.ts: on HERDR_ENV=1 + HERDR_SOCKET_PATH + HERDR_PANE_ID it reports pane.report_agent with source: "herdr:pi", agent: "prime-agent", state: working|blocked|idle, monotonic seq, and agent_session_path/agent_session_id; pane.release_agent on quit. It defers to herdr's file-based pi integration when one is installed.
• herdr accepts these reports: normalize_reported_agent_label falls back to the raw label, full_lifecycle_hook_authority returns false → route_full_lifecycle_hook_report returns Accept, and the pane becomes a hook-authority agent terminal. Empirically: a prime-agent TUI launched in a herdr pane appears in agent list as agent: "prime-agent", agent_status: "idle", renamable, agent wait works.
• Blocked today: agent prompt / agent send-keys require terminal.effective_known_agent() (label must parse to the closed Agent enum); agent.start --kind pa fails at parse_agent_label. server.agent_manifests registers screen-detection rules only and cannot register new kinds (maintainer confirmed in Register prime-agent in the detection registry — reference integration is not promptable (agent prompt/explain fail) #2633).

Requested API surface (minimal)

1. src/detect/mod.rs — Agent enum: add PrimeAgent; agent_label → "prime-agent"; lookup_agent aliases "pa" | "prime-agent"; extend Agent::ALL.
2. agent start canonical executable mapping: interactive_agent_executable(Agent::PrimeAgent) → prime-agent (already on PATH via npm/mise).
3. full_lifecycle_hook_authority entry for the lifecycle-authority class — the pair herdr actually sees is ("herdr:pi", "prime-agent") (the extension hardcodes source "herdr:pi"); once a pa kind exists the extension should switch to source: "herdr:pa" and the entry would be ("herdr:pa", "prime-agent").
4. (Optional) SCREEN_MANIFEST_AGENTS + a detection manifest rule — unnecessary while the socket reporter is active, useful for seats without env.

Items 1–2 unblock agent start --kind pa + agent prompt/send-keys/explain; item 3 opts the pane out of screen/process fallback.

Impact

flywheel (swarm orchestrator) can then spawn prime-agent seats through the standard herdr agent start path instead of the current pane-run fallback; today the orchestrator must bypass agent.start because of the closed enum, and prompt-driven workflows are impossible until the kind exists.