chore: add AGENTS.md, helper scripts, codex templates, and CI

AGENTS.md

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+Codex Agent Guide for SubManager
+
+Scope
+- Applies to the entire repository unless a more specific rule exists deeper in the tree.
+- Precedence: project AGENTS.md > project .claude/agents > user ~/.claude > default Codex CLI rules. Direct system/developer instructions always win.
+
+Goals
+- Accelerate small, safe changes with consistent quality.
+- Keep diffs minimal, focused, and aligned with Flutter best practices.
+
+Guardrails
+- Workspace only: modify files within this repo. Ask before adding dependencies or using network.
+- Safety: avoid destructive actions (file deletions, rewrites, config changes) unless explicitly requested.
+- Responses: be concise; code first, short rationale after. If uncertain, prefix with "Uncertain:". If multiple viable solutions, show the top 2 briefly.
+- Planning: for multi‑step tasks, maintain an update_plan with exactly one in_progress step.
+
+Coding Standards
+- Language: Dart/Flutter (SDK >= 3.0). Respect `analysis_options.yaml` (flutter_lints baseline).
+- Style/format: use `dart format .` and keep changes minimal. Avoid one‑letter variable names; avoid inline comments unless requested.
+- Structure: follow existing file/module patterns and naming. Do not introduce new frameworks or architectural shifts without approval.
+- Tests: add or update tests when behavior changes or bugs are fixed (if feasible). Keep tests scoped to the change.
+
+Validation
+- Always run local checks via `scripts/check.sh` before proposing completion:
+  - formatting check: `dart format --set-exit-if-changed .`
+  - static analysis: `flutter analyze`
+  - unit/widget tests: `flutter test` (ok if none exist)
+- UI changes: include brief description of visual impact; screenshots if readily available by the user.
+
+Sensitive Areas (require explicit approval)
+- Android/iOS/macOS build configs, signing, bundle identifiers, Gradle/Kotlin/Swift project settings.
+- Dependency graph changes (pubspec.yaml add/remove/upgrade).
+- Network access, calling external APIs, or adding secrets.
+
+Operational Conventions
+- Branch naming: `codex/<type>-<slug>` (e.g., `codex/fix-url-matcher`).
+- Commits: Conventional Commits preferred (e.g., `fix: correct url matching for X`).
+- PR description template:
+  - Summary: what/why
+  - Changes: key files and decisions
+  - Validation: how verified (analyze/tests/manual)
+  - Risk & Rollback: potential impact and quick rollback steps
+
+Task Template (author-provided)
+---
+Next: <what to do>
+Complexity: simple | medium | complex
+
+Context
+- Problem / goal:
+- Constraints / non‑goals:
+- Repro or commands:
+
+Done When
+- [ ] Behavior verified (`scripts/check.sh` passes)
+- [ ] Tests/docs updated if applicable
+---
+
+Commands
+- Lint/analyze/tests: `scripts/check.sh`
+- Auto‑format: `scripts/fix.sh`
+
+References & External Facts
+- Prefer official docs and code‑local references. If citing sources, include plain URLs or file paths in PR descriptions (avoid footnote citation syntaxes).
+
+Notes from ~/.claude (adapted)
+- Few‑shot examples improve accuracy; include small before/after or sample input→output when helpful.
+- Use structured thinking internally; present only concise, actionable outputs here.
+