docs: contributing guide for adding a hook-only agent

[njbrake]

Note: this issue was drafted by Claude via back-and-forth with @njbrake. The reasoning and decisions are his; the prose is Claude's.

AGENTS.md documents the settings-field wiring required for new SandboxConfig / WorktreeConfig fields. There's no equivalent for adding an agent that uses a custom hook schema (settl, hermes, kiro), even though that path now has five touchpoints:

1. AGENTS array entry in src/agents.rs (with hook_config: None)
2. Sidecar installer + uninstaller in src/hooks/mod.rs, registered in uninstall_all_hooks
3. Host install branch in install_agent_status_hooks (src/session/instance.rs)
4. Tool name added to the env-prefix list in status_hook_env_prefix (same file)
5. Sandbox install branch + volume mount gate in build_container_config (src/session/container_config.rs)

PR #958's first revision missed step 5, which is exactly the kind of mistake docs would prevent. A short "Adding an agent with a custom hook schema" subsection in AGENTS.md, with a checklist of the five touchpoints and a pointer to install_hermes_hooks as the reference implementation, should be enough.

Pairs naturally with the refactor in #960; if that lands first, the docs section gets shorter.

[njbrake]

Note: this comment was drafted by Claude via back-and-forth with @njbrake. The reasoning and decisions are his; the prose is Claude's.

Addressed by #959, which adds docs/development/adding-agents.md covering all five touchpoints (agent registry, sidecar installer, host install branch, env-prefix list, sandbox install branch) plus a "Common Pitfalls" section that bakes in lessons from #958 review. Will close once #959 merges.

[Seluj78]

@njbrake This issue should be closed ? :)