docs: add agent knowledge base, cross-repo contract and skills

[MaxDac]

Summary

Externalises the knowledge needed to refactor Privee and coordinate with MaxDac/PriveeApp and its F-Droid releases, for Claude Code and GitHub Copilot.

• Adds stack/version, architecture and server–Android contract docs, with compatibility, wake-up-only push and libsignal alignment guidance.
• Adds tool-specific agent instructions and mirrored skills: api-change, local-dev-stack, deploy-privee, libsignal-wasm-upgrade, security-change-review. Preserves the generic Phoenix guidelines in AGENTS.md and links the knowledge base from the README.
• Adds a CI job checking that .claude/skills and .github/skills are identical; preserves pull-request, push-to-main and manual CI triggers.
• Merges current main, including Move deployment to PriveeDeploy; Privee runs CI only #174's move to MaxDac/PriveeDeploy. Privee owns source, Dockerfile and CI only; PriveeDeploy owns fly.toml and manual deploy.yml. Removes the superseded deploy skill and uses deploy-privee as the single canonical procedure.
• Corrects all deployment references and the removed README anchor. Agent deployments require successful CI for the full source SHA and explicit user confirmation. Deploy the server before a dependent app release; rollback requires an explicitly chosen older SHA and database migration compatibility.

Validation

• All five mirrored skill files are byte-identical.
• All 110 relative Markdown links/anchors checked across the updated docs and skills resolve.
• CI YAML parses; pull-request, push-to-main and manual triggers remain intact. No local fly.toml or deployment workflow is reintroduced.
• Deployment guidance checked against PriveeDeploy's current README, deploy.yml, fly.toml and merged self-hosting documentation.
• Full CI results are shown in the PR checks. No production deployment is performed.