fix(preview): sanitize channel-derived versions for plugin installs

[anduimagui]

Issue for this PR

Closes #16224

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This fixes preview version generation when the channel/branch name contains / (for example opencode/lucky-forest), which produced a non-semver prerelease label and broke plugin dependency installs in desktop dev/new workspace flows.

The patch sanitizes preview channel/version strings to semver-safe tokens and adds a pluginVersion guard in config install logic so invalid preview versions automatically fall back to *.

How did you verify your code works?

• Ran bun run dev:desktop from repo root; desktop startup proceeded through predev/build and launched without the semver install failure.
• Ran bun test src/version.test.ts in packages/script.
• Ran bun test test/config/config.test.ts -t pluginVersion in packages/opencode.

Screenshots / recordings

N/A (non-UI fix).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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[github-actions]

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