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>

src/session.ts

@@ -194,11 +194,20 @@ function getModeLabel(mode: SessionMode): string {
  * repaint via cursor positioning, so dropping the alt-screen switch is safe —
  * content stays in the normal buffer. Excluded: `shell` (arbitrary programs like
  * vim/less/htop legitimately need the alt screen), `opencode` (renders its own
- * TUI that may rely on it) and `pi` (its default TUI already renders into the
- * MAIN screen with terminal-owned scrollback, so there is nothing to strip — and
- * since pi 0.84.0 the user can switch to a fullscreen TUI at runtime via
- * `/settings`, where the alt screen is load-bearing). Keep parity with the
- * replay-side strip in session-routes.ts.
+ * TUI that may rely on it) and `pi` (below). Keep parity with the replay-side
+ * strip in session-routes.ts.
+ *
+ * ⚠️ Being excluded here does NOT preserve the alt screen. Every excluded mode
+ * falls through to isMuxAltScreenOnlyStripMode(), which strips the alt-screen
+ * toggles too whenever the session is tmux-backed, and pi/opencode ALWAYS are
+ * (both refuse the direct-PTY fallback). What exclusion actually buys is the rest
+ * of the full strip: `\x1b[3J` and the mouse-tracking DECSETs survive. That is the
+ * real reason pi is out: its default TUI renders into the MAIN screen with
+ * terminal-owned scrollback and is mouse-aware, so it is a `3J`/mouse consumer in
+ * a way an Ink TUI repainting in place is not. Consequence to know before
+ * debugging it: pi's runtime-switchable fullscreen TUI (`/settings`, 0.84.0+)
+ * still gets its `?1049h` stripped and paints into the main buffer, exactly like
+ * vim inside a tmux `shell` session.
  */
 export function isAltScreenStripMode(mode: SessionMode): boolean {
   return mode === 'codex' || mode === 'claude' || mode === 'gemini';