add rate limiting

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+# Changelog — 2026-04-01 #01
+
+## Changes
+
+### New: Rate Limiter (`lib/dispatch/adapter/rate_limiter.rb`)
+- Implemented `Dispatch::Adapter::RateLimiter` class with cross-process rate limiting via filesystem locks (`flock`).
+- Supports per-request cooldown (`min_request_interval`) and sliding window limiting (`rate_limit`).
+- State stored as JSON in `{token_path_dir}/copilot_rate_limit` with `0600` permissions.
+- Handles missing, empty, or corrupt state files gracefully.
+- Validates constructor arguments with descriptive `ArgumentError` messages.
+
+### Modified: Copilot Adapter (`lib/dispatch/adapter/copilot.rb`)
+- Added `min_request_interval:` (default `3.0`) and `rate_limit:` (default `nil`) constructor parameters.
+- Instantiates a `RateLimiter` in `initialize`.
+- Calls `@rate_limiter.wait!` before HTTP requests in `chat_non_streaming`, `chat_streaming`, and `list_models`.
+- Added `require_relative "rate_limiter"`.
+
+### Version Bump (`lib/dispatch/adapter/version.rb`)
+- Bumped `VERSION` from `"0.1.0"` to `"0.2.0"`.
+
+### Updated Test (`spec/dispatch/adapter/copilot_spec.rb`)
+- Updated VERSION expectation from `"0.1.0"` to `"0.2.0"`.
+
+### RuboCop Config (`.rubocop.yml`)
+- Disabled `Layout/LineLength` cop.
+- Disabled `Style/Documentation` cop.