Add public SourceBraid launch kit

[patrickschiller]

Summary

• add a single source of truth for SourceBraid launch positioning, audiences, variables, and proof rules
• document GitHub metadata, the maintained topic set, Discussions setup, and a release-description template
• add the coordinated release checklist and a 60-second demo script
• add channel-specific drafts for Product Hunt, Show HN, r/selfhosted, r/PKMS, and r/opensource

Why

SourceBraid already has the product, privacy, release, and contribution documentation needed for an open-source project, but its launch material was not yet versioned or coordinated. This package makes release gates, public claims, channel copy, and follow-up measurement reviewable alongside the code.

Privacy and permissions

This PR changes documentation only. It does not change browser permissions, GitHub scopes, network endpoints, storage formats, deletion behavior, or credential handling.

The launch material explicitly excludes private archive content, credentials, real repository screenshots, and unapproved captured material. Demo and store assets must use synthetic or clearly redistributable content.

Launch TODO

• Apply and verify the documented GitHub topic set
• Review and merge this launch kit
• Align Chrome, plugin, iOS app, and Share Extension versions
• Run the required release, package, and unsigned iOS checks
• Build and inspect allowlisted release artifacts and record their SHA-256 digests
• Publish the coordinated GitHub release
• Publish and verify the selected Chrome Web Store, App Store, and OpenAI plugin destinations
• Complete clean first-run installation, capture, PDF, and archive-search tests
• Record and caption the demo with a private synthetic repository
• Enable GitHub Discussions and publish the welcome discussion
• Run the 15-25 person soft launch and fix blocking onboarding issues
• Publish Show HN, Reddit, and Product Hunt posts on separate days
• Review activation, first search, and seven-day return signals after launch

The detailed gates and owners remain in marketing/LAUNCH_CHECKLIST.md.

Checks

• git diff --check origin/main...HEAD
• verified that the PR contains only the seven new files under marketing/
• verified the eight documented {{...}} launch variables
• scanned the launch material for common credential and private-key patterns
• confirmed the commit contains the required DCO Signed-off-by: trailer

Code and platform test suites were not run because this PR adds Markdown launch material only.