docs: add AGENTS.md

AGENTS.md

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+# AGENTS.md
+
+This repo is a competitive programming reference library (ICPC, CF, BOJ, etc).
+When adding or editing code here, optimize for contest usage first.
+
+## Core goals (in priority order)
+1. **Quick to type and paste**: short, practical APIs and file structure.
+2. **Fast and memory efficient**: proven time complexity, low constants, minimal allocations.
+3. **Readable under pressure**: simple control flow, obvious invariants, and friendly comments.
+4. **Consistent style**: match the rules in this file and existing patterns in this repo.
+5. **Template-friendly**: easy to reuse across problems with minimal edits.
+
+## Repo standard code style (required)
+### Naming
+- **Struct names**: `PascalCase` (capitalized).
+  - Example: `struct FenwickTree { ... };`
+- **Constants** (e.g., `constexpr`, global `const`, fixed parameters): `UPPER_SNAKE_CASE`.
+  - Examples: `constexpr int MOD = 1e9 + 7;`, `const int INF = 1e9;`
+- **Everything else** uses **lowercase `snake_case`**:
+  - namespaces, functions, variables, file names
+  - Examples: `namespace fast_io`, `add_edge`, `max_flow`, `solve_case`, `pref_sum`
+
+### Types
+- Prefer `ll` by default to reduce overflow debugging.
+- Use `int` only when clearly safe and beneficial (memory, bitset indexing, array indices, tight constraints).
+- Avoid implicit narrowing conversions. Cast explicitly at boundaries when mixing types.
+
+## Coding rules
+- Prefer straightforward implementations over heavy abstractions.
+- Avoid unnecessary dynamic polymorphism, complex metaprogramming, and over-engineered designs.
+- Public-facing helpers should be easy to understand without reading 5 other files.
+- Template common patterns (I/O, loops, small utilities), but keep templates minimal.
+
+## Comments and documentation (required)
+Write friendly, high-signal documentation in a consistent format.
+
+### 1) Module header (for each reusable component)
+Add a short header comment near the top:
+- What it does
+- Complexity (time, memory)
+- Constraints / gotchas (short)
+- Optional: where it was verified (contest / problem ID)
+
+### 2) Inline comments (throughout the code)
+Comment generously with short, consistent one-liners:
+- intent (`// goal: ...`)
+- invariants (`// invariant: ...` = must stay true throughout the loop/algorithm)
+- edge cases (`// edge: ...`)
+- tricky reasoning (`// why: ...`)
+- usage (`// usage: ...`)
+
+Prefer many small comments over a few long paragraphs.
+
+## Formatting (required)
+After any change, run:
+- `scripts/format.sh`
+
+Do not commit unformatted code.
+
+## Consistency checklist
+Before finishing a change:
+- Style matches the rules above (naming, layout, aliases).
+- The API is minimal and pasteable.
+- No extra dependencies unless clearly worth it.
+- Complexity and constraints are stated.
+- Comments explain intent/invariants/edges without being verbose.
+- The code is not longer than it needs to be.