docs(skill): document the per-CLI availability probes, and guard the family

`GET /api/pi/status` shipped undocumented in the agent skill, and only a human
reading the doc noticed. Turns out none of its five siblings were documented
either, so this adds the whole family in one place: spawning with a mode whose
CLI is absent fails with OPERATION_FAILED rather than falling back, which is
exactly what an agent picking a backend it did not choose needs to know. Pi's
extra `.data.version` is called out, since a false `available:false` there means
an unrelated `pi` is in front on PATH.

On whether the endpoint scanner should also check registered-to-documented:
measured, and NO for the general case. The skill documents 34 of 217 registered
endpoints deliberately (it is an agent guide, not an API reference), so a blanket
reverse check needs a 183-entry allowlist that would fail CI on unrelated route
work and get appended to mechanically, which is worse than the gap it closes.
Grouping by path shape does not save it either: the families that yields are
things like `DELETE /api/<any>/:id`, lumping cases, webviews and docker hosts
together, and it would not have caught this gap anyway (the family had zero
documented members).

What IS cheap is a family the schema can enumerate with no allowlist: the new
assertion derives the agent modes from the Zod enum and requires each one's
`/api/<mode>/status` to be documented, so a seventh backend fails here until it
is. The sibling scanner still proves the other direction, that nothing documented
is a 404. Both mutation-checked: dropping pi's probe fails the new guard, and
documenting a nonexistent probe fails the old one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

skills/codeman/reference/endpoints.md

@@ -338,6 +338,16 @@ ESC=$(printf '\033')
 `.data.{sessionId, caseName, casePath}`. Creates the case directory (a real directory
 on the user's disk) if missing, do not retry it in a loop, and remember the name.
 
+⚠️ A `mode` whose CLI is **not installed on the server** fails the spawn with
+`OPERATION_FAILED`; it never falls back to claude. Probe first whenever you did not pick
+the mode yourself: `GET /api/v1/claude/status`, `GET /api/v1/opencode/status`,
+`GET /api/v1/codex/status`, `GET /api/v1/gemini/status`, `GET /api/v1/antigravity/status`
+and `GET /api/v1/pi/status` each return `.data.{available, path}` (no session needed).
+Pi's also carries `.data.version`, because `pi` is a short generic name that an unrelated
+binary on `$PATH` can shadow: the resolver rejects one whose `--version` is not
+semver-shaped, so `available:false` there can mean "a different `pi` is in front" rather
+than "nothing is installed". `shell` has no CLI to probe.
+
 ⚠️ **Branch on `.success` before reading `.data.sessionId`.** On any failure the field
 is absent, `jq -r` prints the literal string `null`, and every later call then targets
 `/api/v1/sessions/null`, burning the full readiness budget and reporting jq noise