docs: add CLAUDE.md with project conventions

CLAUDE.md

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+# CLAUDE.md — Slack::WebHook
+
+## What is this
+
+A CPAN Perl module for sending Slack webhook notifications with preset colors and layouts.
+~250 lines of Perl. Simple, focused, stable.
+
+## Project structure
+
+```
+lib/Slack/WebHook.pm   — The module (all logic lives here)
+t/hooks.t              — Test suite (Test2-based, mocks HTTP::Tiny)
+examples/              — Example scripts (embedded in POD via Dist::Zilla InsertExample)
+dist.ini               — Dist::Zilla config (drives build, release, Makefile.PL generation)
+cpanfile               — Dependencies (used by CI and cpanm)
+Makefile.PL            — Auto-generated by dist.ini, committed for CI convenience
+.github/workflows/     — GitHub Actions CI
+```
+
+## Commands
+
+```bash
+# Run tests locally
+prove -Ilib -v t/hooks.t
+
+# Install test dependencies
+cpanm --installdeps .
+```
+
+## Architecture
+
+Call flow: `post_*()` → `_notify()` → `notify_slack()` → `_http_post()`
+
+- **`post()`** — Raw message, bypasses attachment formatting
+- **`post_ok/warning/error/info()`** — Colored attachment wrappers
+- **`post_start/post_end()`** — Timer pair (elapsed time appended to message)
+- **`_notify()`** — Normalizes args (string or key-value pairs) → `notify_slack()`
+- **`notify_slack()`** — Builds attachment structure with color, title, text, mrkdwn_in
+- **`_http_post()`** — UTF-8 auto-detect + JSON encode + HTTP::Tiny post
+
+## Key conventions
+
+- **OO via Simple::Accessor** — `new()` calls `$self->$key($val)` for each constructor arg.
+  Accessor names must be **lowercase** (e.g., `url`, not `URL`).
+- **Build system**: Dist::Zilla. `Makefile.PL` and `README.md` are auto-generated.
+  Edit `dist.ini` for dependency or metadata changes, not Makefile.PL directly.
+- **Examples**: Files in `examples/` are embedded in POD via `[InsertExample]`.
+  They must have `use Slack::WebHook ();` and be syntactically valid Perl.
+- **Minimum Perl**: 5.010 (declared in dist.ini). Don't use features above 5.10.
+
+## Testing
+
+- Tests mock `HTTP::Tiny::post_form` to capture payloads — no real HTTP calls
+- `http_post_was_called_with()` is the main assertion helper: decodes JSON payload and compares
+- `Test2::Plugin::NoWarnings` is active — unexpected warnings fail tests
+- When testing warning paths, use `local $SIG{__WARN__}` to capture and test explicitly
+- Mock must return `{ success => 1 }` — `_http_post()` warns on failure responses
+
+## Dependencies
+
+Runtime: `Simple::Accessor`, `HTTP::Tiny` (>= 0.076), `JSON::XS`, `Encode`
+Test: `Test2::Bundle::Extended`, `Test2::Tools::Explain`, `Test2::Plugin::NoWarnings`, `Test::MockModule`
+
+## Git workflow
+
+- Branch prefix: `koan.atoomic/*`
+- Never commit to master directly
+- PRs target master, always draft