fix(fleet): verify via pane-change diff + non-resizing watch

Blocker fix: send --verify now captures a BEFORE snapshot immediately
before the send and an AFTER snapshot after the delay, then uses
classifySendResult(before, after) to classify. A wedged pane showing
stale non-empty content is no longer falsely reported as 'accepted' —
BEFORE==AFTER maps to 'unverifiable' (exit 1, "no pane change after
send"). Blank AFTER still fails closed as 'unverifiable'. Only
AFTER != BEFORE without a draft suffix counts as 'accepted' (exit 0).

Should-fix: agent watch now uses a GROUPED VIEWER SESSION instead of a
bare 'tmux attach -r' against the agent session. A bare attach lets the
viewer terminal shrink the agent's window; a grouped session has
independent sizing so the agent's window is never affected.
Sequence: new-session -d -t '=<agent>' -s '<agent>-watch-<pid>' (runner),
attach -r to viewer session (interactiveRunner), kill-session on detach
(runner). New builder functions exported: buildAgentWatchCreateViewerCommand,
buildAgentWatchAttachCommand, buildAgentWatchKillViewerCommand,
buildViewerSessionName. buildAgentWatchCommand kept but deprecated.

New exports: classifySendResult(before, after) — the testable classifier.

Tests added:
- classifySendResult unit suite (6 cases): accepted/draft/unverifiable/
  stale-pane/both-blank/before-blank-after-response
- send --verify regression: stale (before==after non-empty) => exit 1
- send --verify regression: blank AFTER => exit 1
- send --verify regression: draft after pane change => exit 1
- send --verify regression: changed non-draft => exit 0
- send --verify: 3-call sequence assertion (before-capture, send, after-capture)
- watch dispatch: grouped viewer session created/attached/killed; no bare
  attach against agent session; viewer name matches <agent>-watch-<pid>

PRD Known-limitations updated: pane-change check rationale, Phase-3
heartbeat-ack requirement, grouped-session watch design.

All gates pass: pnpm typecheck, pnpm lint, pnpm --filter @mosaicstack/mosaic test
(382 tests, 74 fleet), prettier --check.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RMoEx7hfdFGjUiCHuN1RRi

docs/fleet/PRD.md

@@ -80,13 +80,25 @@ observability and no safe way to watch a session.
 - **Verify heuristic is best-effort:** `agent send --verify` uses a `>` -prefix draft
   heuristic that is specific to pi/claude TUIs. Draft detection for codex and opencode
   TUIs is best-effort only; those runtimes may not use the same input-line indicator.
-- **Blank capture fails closed:** Full-screen TUIs (claude, codex, opencode, pi) render
-  blank for `tmux capture-pane`. When the captured output is empty, `send --verify`
+- **Pane-change check is the best Phase-2 signal:** `agent send --verify` compares a
+  BEFORE snapshot (captured immediately before the send) to an AFTER snapshot (captured
+  after the send delay). A pane that changed and does not end in a draft line is reported
+  as 'accepted'. A pane that did not change — including a wedged pane showing stale
+  non-empty content — is reported 'unverifiable' (exit 1, "no pane change after send").
+  Definitive acceptance ultimately requires a runtime acknowledgement (Phase-3
+  heartbeat-ack); the pane-change check is the best signal available against an opaque
+  TUI for Phase-2.
+- **Blank AFTER capture fails closed:** Full-screen TUIs (claude, codex, opencode, pi)
+  render blank for `tmux capture-pane`. When the AFTER snapshot is empty, `send --verify`
   returns non-zero with an "unverifiable" message rather than silently succeeding. This
   is an intentional fail-closed design (FR-5).
-- **`agent watch` requires TTY passthrough:** `tmux attach` is interactive and must be
-  run with inherited stdio. It cannot be captured through a pipe. Tests inject a fake
-  `interactiveRunner`; the real implementation spawns with `stdio: 'inherit'`.
+- **`agent watch` uses a grouped viewer session:** `tmux attach -r` directly against the
+  agent session lets the viewer terminal shrink the agent's window. `agent watch` instead
+  creates a throwaway grouped session (`tmux new-session -d -t '=<agent>' -s
+'<agent>-watch-<pid>'`), attaches read-only to that session, and kills it on detach.
+  The grouped session shares the agent's windows but has independent sizing, so the
+  agent's window is never affected. `tmux attach` is still interactive and requires
+  inherited stdio; the `interactiveRunner` handles TTY passthrough.
 
 ## Surfaces & parity (MVP-X1)