chore: remove internal materials from public repository

[hepinga]

Outcome

Keeps the public repository focused on user-facing and contributor-relevant material.

• removes internal PRDs, research archives, requirement notes, service/account planning, repository makeover research, maintainer release runbooks, local agent instructions, and unpublished release drafts
• removes an account-level hosting project identifier from tracked files
• anonymizes personal test fixtures
• removes internal-runbook links from both READMEs and keeps only public release-integrity requirements
• adds repository-boundary guidance and PR checks to prevent internal material from returning
• keeps published v0.9.0 notes, README assets, privacy/security policies, source, build scripts, and contributor documentation

Verification

• ./scripts/test.sh: 26 tests, 0 failures (1 expected live-data skip)
• ./scripts/build.sh 0.10.0 3: succeeded in isolated worktree
• website npm test: 17/17 passed
• website npm run lint: passed
• GitHub makeover audit: PASS 100/100
• README relative links: 26 checked, 0 broken
• targeted secret/private-identifier scan: no findings
• git diff --check

This cleans the current public tree. It does not rewrite existing Git history.