docs: document CLI version pinning

[factory-davidgu]

Description

Document how to pin the Droid CLI through npm, the built-in updater, or direct verified binary downloads. Clarify update command flags and how automatic updates interact with pinned standalone and npm installations.

How Has This Been Tested?

• Ran git diff --check
• Verified droid@0.174.0 exists on npm
• Verified the Linux x64 binary and checksum endpoints for v0.174.0 return HTTP 200

[factory-droid]

Droid finished @factory-davidgu's task —— View job

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The documentation updates are clear and add useful guidance for pinning and installing specific CLI versions. The main remaining gaps are security and operational clarity around installation trust (especially curl | sh and same-host checksums) and a more explicit upgrade workflow when auto-updates are disabled.

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[factory-droid]

[P1] Document how to intentionally upgrade when auto-updates are disabled

This section says that setting FACTORY_DROID_AUTO_UPDATE_ENABLED=false disables automatic updates and that droid update will not install another version, but it does not explain the intentional upgrade path for pinned standalone installs. Add a sentence telling readers to temporarily unset or flip the variable (or otherwise re-enable updates) when they want to run droid update --version <x>, then re-disable auto-updates to remain pinned.