fix(pi): align the doctor with the pi resolver, correct the strip rationale, update the skill

Second review pass on #282, the three items left open after f4dcfbe.

1. `codeman doctor` and the run mode disagreed about pi. The registry entry
   accepted a bare `which pi` hit while pi-cli-resolver demanded semver-shaped
   `--version` output, so the Dependencies panel could report an installed Pi CLI
   on a box where Run Pi stays hidden, which reads as a broken mode rather than a
   missing install. Both sides now share one exported PI_VERSION_REGEX, and
   PathResolver gains an opt-in `requireVersionMatch` so a binary that fails the
   shape check is reported MISSING instead of installed-with-unknown-version.
   Only pi sets it; every other tool keeps its current behaviour.

2. The isAltScreenStripMode comment justified excluding pi with "the alt screen
   is load-bearing for its fullscreen TUI". That is not what exclusion does: pi
   is tmux-backed, so it falls through to isMuxAltScreenOnlyStripMode, which
   strips the alt-screen toggles anyway. What exclusion actually preserves is
   `\x1b[3J` and the mouse DECSETs, which is the real reason (pi renders into the
   main screen and is mouse-aware). Comment and changeset now say that, and state
   the consequence: fullscreen pi paints into the main buffer, like vim in a tmux
   shell session.

3. skills/codeman still enumerated the five pre-pi modes in nine places, telling
   agents a backend does not exist and understating class-wide caveats by one
   mode. All updated, plus stale session.ts line references refreshed.

Tests: a new static guard derives the mode set from the Zod schema (not a copy)
and fails when a skill enumeration lists a partial set of external CLIs, verified
by mutation. It also documents the one legitimate exception it found: the "writes
no transcript" lists drop codex, which does write a rollout Codeman reads back.
Plus doctor cases for an unrelated `pi` on PATH and registry/resolver regex parity.

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

skills/codeman/reference/messaging.md

@@ -56,7 +56,7 @@ own head: the worker enforcing the cap is the one who has to be told about it.
 | synchronize on end of turn | HTTP `wait until=stop` (fires for message-initiated turns too, verified live) |
 | liveness / death check | HTTP `wait?until=exit` |
 | interrupt a running turn (break-glass) | HTTP input, a bare `\x1b` with no `\r` |
-| non-claude modes (`shell`/`opencode`/`codex`/`gemini`/`antigravity`) | HTTP only (no other CLI has messaging) |
+| non-claude modes (`shell`/`opencode`/`codex`/`gemini`/`antigravity`/`pi`) | HTTP only (no other CLI has messaging) |
 | delete | HTTP, via SKILL.md's `delete_session` guard |
 
 ## Availability: probe, never assume
@@ -345,7 +345,7 @@ Without a break-glass, a pair with a bad brief is a token bonfire with no off sw
 
 ### Mixed fleets: the pairing matrix
 
-Non-claude workers (`shell`, `opencode`, `codex`, `gemini`, `antigravity`) cannot be peers
+Non-claude workers (`shell`, `opencode`, `codex`, `gemini`, `antigravity`, `pi`) cannot be peers
 at all; no other CLI has this feature. Their tasks route over HTTP, and you never mention
 messaging in their briefs. The claude half of the fleet can use messaging among itself,
 subject to the namespace rule: **messaging works between two sessions that share one