chore: add CONTRIBUTING and SECURITY guidelines to improve project documentation

CONTRIBUTING.md

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+# Contributing to pyfragment
+
+## Development setup
+
+```bash
+git clone https://github.com/bohd4nx/pyfragment.git
+cd pyfragment
+pip install -e ".[dev]"
+```
+
+## Running checks
+
+```bash
+# Lint and format
+ruff check . --fix && ruff format .
+
+# Type check
+mypy . --explicit-package-bases
+
+# Tests
+pytest
+```
+
+All three must pass before opening a PR.
+
+## Project structure
+
+```
+pyfragment/
+  client.py           — FragmentClient (public entry point)
+  enums.py            — ApiProvider, PaymentMethod, WalletVersion
+  exceptions.py       — exception hierarchy
+  core/               — constants, validation helpers
+  domains/            — one package per feature domain
+    ads/              — recharge_ads, topup_gram
+    anonymous_numbers/— get_login_code, toggle_login_codes, terminate_sessions
+    giveaways/        — giveaway_stars, giveaway_premium
+    marketplace/      — search_usernames, search_numbers, search_gifts
+    purchases/        — purchase_stars, purchase_premium
+  services/           — shared infrastructure services
+    cookies/          — browser cookie extraction (models + service)
+    tonapi/           — wallet info, transaction signing (tonapi/toncenter)
+tests/                — unit tests (pytest)
+examples/             — runnable usage examples (excluded from CI)
+```
+
+## Conventions
+
+- All public async methods live on `FragmentClient` and delegate to a domain service.
+- Domain functions receive a `FragmentClient` instance, never raw HTTP clients.
+- Patch targets in tests use the module where the name is **defined**, e.g. `pyfragment.services.tonapi.transaction._make_ton_client`.
+- Versioning follows [CalVer](https://calver.org/): `YYYY.MINOR.MICRO`. Bump in `pyproject.toml`; tag as `vYYYY.MINOR.MICRO`.
+
+## Pull requests
+
+- Keep PRs focused — one feature or fix per PR.
+- Update `CHANGELOG.md` under `[Unreleased]`.
+- Add or update tests for any changed behaviour.

SECURITY.md

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+# Security Policy
+
+## Reporting a vulnerability
+
+Please **do not** open a public GitHub issue for security vulnerabilities.
+
+Report them privately via GitHub's [Security Advisory](https://github.com/bohd4nx/pyfragment/security/advisories/new) feature, or contact the maintainer directly at [@bohd4nx](https://t.me/bohd4nx) on Telegram.
+
+Include:
+
+- A description of the vulnerability and its potential impact.
+- Steps to reproduce or a proof-of-concept.
+- Affected versions.
+
+You will receive a response within 72 hours. Once the fix is released, the advisory will be published.
+
+## Scope
+
+This library handles sensitive credentials (GRAM (ex TON) seed phrases, Fragment session cookies, Tonapi keys). Please treat any finding that could expose or misuse these credentials as high severity.