docs(AGENTS): add dev-flow guidance (version-pinning, self-review, mistake-logging, clean-stop) + Kotlin style rules

[NiDrZero]

What this changes

Documentation only — adds guidance to AGENTS.md. No code, no build/CI impact. 74 insertions, 0 deletions; nothing existing was modified.

Four dev-flow principles (adapted from GAIA Code, alexey-max-fedorov/gaia-ai)

• Pin versions from the source of truth (Safety Checklist) — take dependency versions from the official registry/release page, not a search snippet.
• Self-review before opening a PR (Review Rules) — read the whole change yourself before asking for review.
• Record mistakes as forward-looking rules (Review Rules) — when a fix lands, log the lesson in the same session.
• Stopping cleanly (Continuation & Focus) — leave a short done / current-state / next-step note when pausing.

Five Kotlin idioms (adapted from jbaruch/kotlin-tutor, Android-fit subset)

New Kotlin Style Rules section: prefer val; use String? over Optional; use data class for value types; use scope helpers (let/apply/also) only when they aid clarity; prefer extension functions over *Utils classes.

Explicitly noted as not adopted / shelved:

• StateFlow over polling — shelved until the app-shell phase (no code needs it yet).
• Kotest over JUnit — deliberately skipped; would break the current passing JUnit CI for no gain on a solo project.
• All of kotlin-tutor's server/ML stack rules (Ktor, DJL, JavaCV, etc.) — omitted as wrong-fit for Android.

Every addition is written plain-English-first per the AGENTS.md communication rules.

Checks

• Only AGENTS.md staged/changed (verified).
• No local.properties, *.so, *.tar.xz, or gradlew staged (verified).
• Left open for review; nothing merged.