docs: add agent guidance

[mkaltner]

What changed

• add repository-level AGENTS.md guidance for Arcane Android
• document the Compose architecture, SDK boundary, implementation conventions, and verification commands
• record the collaborator workflow and publishing safety boundaries

Why

Future coding-agent sessions need durable repository context so they preserve the existing architecture, use libarcane-kotlin instead of duplicating API code, and run the same checks as CI.

Impact

Documentation only. There are no application or build-system changes.

Validation

• git diff --cached --check
• verified every referenced repository path exists
• reviewed the staged scope to confirm only AGENTS.md is included

AI assistance

OpenAI Codex drafted and validated the guidance. This draft PR remains available for human review and editing.

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Greptile Summary

This PR adds agent and parity-planning documentation for Arcane Android. The main changes are:

• Repository-level AGENTS.md guidance for architecture, SDK use, and verification.
• A new iOS-to-Android gap analysis document.
• A new Android iOS-parity task list derived from the gap analysis.

Confidence Score: 5/5

This looks safe to merge after the backlog consistency issues are cleaned up.

• The changed files are documentation only.
• The main repo guidance matches the checked Android layout and Gradle commands.
• The parity backlog misses some work from the gap analysis and places one urgent validation task under a later phase.

docs/ios-parity-task-list.md

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
docs/ios-parity-task-list.md:75
**Missing Roadmap Tasks**

The Phase 0 backlog does not track two items that the paired gap analysis lists as immediate work: the unreachable environment list/detail/test surface and the Light/Dark/Auto appearance setting. A contributor planning from this backlog can leave those fixes unassigned, so the documented roadmap and the actionable task list diverge.

### Issue 2 of 2
docs/ios-parity-task-list.md:196-197
**Hidden Correctness Task**

`PAR-101` is marked as immediate correctness work, but it sits under the Phase 1 daily-workflow section. Planning that handles only Phase 0 first can skip the destructive prune validation even though the task itself marks that work as urgent.

_{Reviews (1): Last reviewed commit: "docs: add parity backlog checkboxes" | Re-trigger Greptile}

Greptile also left 2 inline comments on this PR.

[greptile-apps]

Missing Roadmap Tasks

The Phase 0 backlog does not track two items that the paired gap analysis lists as immediate work: the unreachable environment list/detail/test surface and the Light/Dark/Auto appearance setting. A contributor planning from this backlog can leave those fixes unassigned, so the documented roadmap and the actionable task list diverge.

Prompt To Fix With AI

This is a comment left during a code review.
Path: docs/ios-parity-task-list.md
Line: 75

Comment:
**Missing Roadmap Tasks**

The Phase 0 backlog does not track two items that the paired gap analysis lists as immediate work: the unreachable environment list/detail/test surface and the Light/Dark/Auto appearance setting. A contributor planning from this backlog can leave those fixes unassigned, so the documented roadmap and the actionable task list diverge.

How can I resolve this? If you propose a fix, please make it concise.

Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

[greptile-apps]

Hidden Correctness Task

PAR-101 is marked as immediate correctness work, but it sits under the Phase 1 daily-workflow section. Planning that handles only Phase 0 first can skip the destructive prune validation even though the task itself marks that work as urgent.

Prompt To Fix With AI

This is a comment left during a code review.
Path: docs/ios-parity-task-list.md
Line: 196-197

Comment:
**Hidden Correctness Task**

`PAR-101` is marked as immediate correctness work, but it sits under the Phase 1 daily-workflow section. Planning that handles only Phase 0 first can skip the destructive prune validation even though the task itself marks that work as urgent.

How can I resolve this? If you propose a fix, please make it concise.