[codex] Fix local update version detection

[BobbyWang0120]

Summary

• Update VERSION from 1.3.0 to 1.6.0 so it matches the current Release Please manifest and latest release tag.
• Teach update-system.mjs to read the local version from .release-please-manifest.json first, falling back to VERSION.
• Reuse the same semver parser for local and remote version sources, and document the prevention rule in AGENTS.md.

Why

The latest release is v1.6.0, but VERSION stayed at 1.3.0. Because localVersion() only read VERSION, a fully updated checkout could still report local: 1.3.0, remote: 1.6.0 and show a false update prompt.

Validation

• node update-system.mjs check -> {"status":"up-to-date","local":"1.6.0","remote":"1.6.0"}
• npm run update:check -> same result
• node test-all.mjs --quick -> 64 passed, 0 failed, 15 existing warnings around branding strings

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