docs: simplify alpha.5 download and onboarding

[FernandoAbishai]

Summary

• Reorders the README around a creator-first alpha.5 download and install path.
• Adds the direct macOS Apple Silicon DMG CTA and release verification link.
• Explains the supported packaged runtime, first-transcription model preparation, and truthful local-first/AI behavior.
• Updates the First Export and User Guide to the current Edit a Video and Create Clips flows.
• Documents Find → Review → Prepare → Export, manual Draft clip, optional publishing copy, advanced settings, caption sidecars, and creator-facing export results.

Validation

• Changed files are limited to README.md, docs/FIRST_EXPORT.md, and docs/USER_GUIDE.md.
• alpha.4 references and obsolete Find Best Clips / Export Approved / Open Video File instructions: none.
• Direct alpha.5 DMG CTA and links: verified.
• Relative documentation links: all resolve.
• git diff --check: passed.
• No Developer ID or notarized claim; no Windows/Linux public installer claim.
• No production source, release workflow, release manifest, tag, or published alpha.5 mutation.

No product/runtime/release behavior was changed. Do not merge as part of this task.

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