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/TASKS.md

@@ -58,3 +58,7 @@ Active workstream is **W1 — Federation v1**. Workers should:
 ## F3-m3 — mosaic update re-seeds framework + relaunches agents (#609) — feat/f3-m3-update-reseed
 
 - Status: implemented + tested. Closes R13: `mosaic update` now re-seeds the framework (data-safe MOSAIC_SYNC_ONLY) after the CLI install so shipped launcher/runtime changes activate; `--relaunch` restarts rostered agents; `--no-reseed` opts out. Detail: scratchpads/f3-m3-update-reseed.md.
+
+## Fleet-polish bundle — boot-survival symmetry (#611) — feat/fleet-polish-bundle
+
+- Status: implemented + tested. disable-on-remove (boot-resurrection bug, TDD) + add-enable + init-R5 hard guarantee. 4 new + 147 existing fleet tests green. Detail: scratchpads/fleet-polish-bundle.md.