fix(fleet): verify fails-closed on unverifiable + interactive watch

- isSendAccepted now returns 'accepted' | 'draft' | 'unverifiable' (was bool)
- Blank/empty capture => 'unverifiable' => process.exitCode=1 with distinct
  "could not verify delivery (blank/no response captured)" message; previously
  blank was treated as success, violating FR-5 fail-closed semantics
- Draft line ('^> ') => process.exitCode=1 with "left as unsubmitted draft"
  message; distinct wording from unverifiable case
- agent watch now dispatched through injectable InteractiveRunner (stdio:inherit)
  instead of the capturing CommandRunner; tmux attach requires TTY passthrough
- Default spawnInteractive implementation uses node:child_process spawn with
  stdio:'inherit'; injectable via FleetCommandDeps.interactiveRunner for tests
- Removed buildSystemdIsActiveCommand (dead code — exported but unused)
- Tests: blank=>exitCode=1, draft=>exitCode=1, real response=>exitCode=0,
  watch dispatched through interactiveRunner not capturing runner
- PRD: added "Known limitations" section (heuristic verify, blank fails closed,
  non-pi/claude draft detection is best-effort, watch requires TTY passthrough)
- Code comment on isSendAccepted notes pi/claude-specific draft heuristic

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

docs/fleet/PRD.md

@@ -75,6 +75,19 @@ observability and no safe way to watch a session.
 - Situational: run against the live `mosaic-factory` fleet; capture `fleet ps` output,
   a kill-and-detect cycle, a read-only `watch`, and a `send --verify` pass/fail pair.
 
+## Known limitations
+
+- **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`
+  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'`.
+
 ## Surfaces & parity (MVP-X1)
 
 CLI lands this phase. TUI surface follows in the `packages/mosaic` wizard; webUI in