docs(readme): correct CI workflow list + add AGENTS.md

[Hidden-Node]

Problem

The README's "GitHub Workflows" section (README.md:309-326) made two false claims:

1. References a non-existent release.yml workflow. The repo only has android-ci.yml and release-manual.yml — release.yml was deleted at some point but the README still listed it.

2. Claims android-ci.yml "Checks code quality". It only builds the debug APK (and now runs unit tests after PR fix(android): VpnTileService dispatcher ordering + CI Go 1.25 alignment #17). No lint step exists.

Changes

README.md — replaced the entire "GitHub Workflows" subsection

• android-ci.yml: Accurately describes trigger (push to main/master + PRs), AAR build, debug APK build, unit tests, artifact upload.
• release-manual.yml: Describes workflow_dispatch inputs (tag_name, release_name, make_latest), signing secrets required, release asset upload via softprops/action-gh-release@v2.
• Adds a blockquote at the end explaining there is no release.yml and how to perform tag-triggered releases.

AGENTS.md — new file (repo root)

Operating context for AI agents (opencode, Cursor, Claude Code, etc.):

• What this repo is — project overview, upstream links
• Architecture — layered diagram (UI → ViewModels → VPN Service/Go Core → Network)
• How to build — AAR, debug APK, release AAB
• How to verify — unit tests, lint, compile check, go vet/gofmt
• Conventions — Kotlin (Hilt, coroutines, error handling, logging) and Go (mutex discipline, error wrapping)
• Things that are easy to get wrong — credentials in logs, destructive Room migrations, engine.Stop() panics, cleartext network_security_config.xml, stale Go mobile AAR
• Running the plans in plans/ — references the plans/ directory index

Eliminates redundant recon work for future agent-assisted development sessions.