fix(fleet): boot-survival symmetry — disable-on-remove + add-enable + init-R5 (#611)

Codex symmetry-gap finding. Three fixes completing add/remove boot-survival
symmetry:

1. disable-on-remove (BUG, TDD): fleet remove stopped + deleted roster/env/
   heartbeat but never disabled the systemd unit, so a removed-but-enabled
   mosaic-agent@NAME.service could resurrect on reboot pointing at deleted
   config. Add buildSystemdDisableCommand + disable in remove (best-effort,
   gated on !--keep-files).
2. add-enable: fleet add now enables the new agent's unit for boot-survival
   (best-effort, independent of --start).
3. init-R5 guarantee: fleet init --write now fails hard when a non-minimal
   profile lacks exactly one orchestrator (was a soft warning); the sanctioned
   no-orchestrator 'minimal' preset is still allowed.

Verified: 4 new tests (disable builder; remove-invokes-disable; add-invokes-
enable; init general → exactly 1 orchestrator) + 147 existing fleet tests green
(151 total); tsc/eslint/prettier clean.

Refs #611

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EsgTQzV5YUGk1JtCLP4B83

docs/scratchpads/fleet-polish-bundle.md

@@ -0,0 +1,20 @@
+# Fleet-polish bundle — boot-survival symmetry (#611)
+
+- **Issue:** #611 · **Branch:** `feat/fleet-polish-bundle` · From the Lead's Codex symmetry-gap finding.
+
+## Three fixes
+
+1. **disable-on-remove (BUG, TDD).** `fleet remove` stopped + deleted roster/env/heartbeat but never
+   `systemctl --user disable mosaic-agent@NAME.service` → a removed-but-enabled unit could resurrect on
+   reboot pointing at deleted config. Fix: `buildSystemdDisableCommand` + disable in `remove`
+   (best-effort, gated on !--keep-files).
+2. **add-enable.** `fleet add` now enables the new agent's unit for boot-survival (best-effort,
+   independent of --start) — symmetry with disable-on-remove.
+3. **init-R5 guarantee.** `fleet init --write` now FAILS HARD when a non-minimal profile doesn't yield
+   exactly one orchestrator (was a soft warning). `minimal` (sanctioned no-orchestrator) still allowed.
+
+## Verification
+
+- 4 new tests (disable builder; remove-invokes-disable; add-invokes-enable; init general → exactly 1
+  orchestrator) + 147 existing fleet tests green (151 total). tsc/eslint/prettier clean.
+- TDD on the disable bug per contract.