Clarify Cloud env persistence in AGENTS.md (snapshot vs install command)

[Vercantez]

Summary

Corrects over-promising wording in the ## Cursor Cloud specific instructions section added earlier. The prior text stated tools were "already installed in the snapshot" and "persist across sessions" as facts, but per the Cursor Cloud docs that persistence only holds if a base environment snapshot is saved (an explicit action) or the tools are baked into a .cursor/environment.json Dockerfile/install command. This run is currently just-in-time (environment: null), so those claims weren't guaranteed.

Changes (docs only)

• Adds a "How persistence works here (read first)" note: Bun, Docker, Playwright, and .dev.vars persist only via a saved snapshot or the environment Dockerfile/install command; the install command (bun install --frozen-lockfile) alone does not recreate them and assumes Bun already exists; daemons like dockerd are never snapshotted and must be restarted each run.
• Softens the .dev.vars, Playwright, and Bun bullets from "already in the snapshot" to "installed during setup; persists only if the snapshot was saved (else reinstall/recreate)."
• Docker subsection: notes Docker persistence is snapshot/Dockerfile-dependent, and that the durable way to start the daemon is the environment start command (sudo service docker start), not just manual sudo dockerd.
• Clarifies Docker belongs in the base snapshot/Dockerfile + start command, not the per-boot install command.

No application code changed.