feat: prepare agen crates for publication

crates/agen/docs/requirements.md

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+# agen requirements
+
+## R1: Turn execution and continuation
+
+- `Engine::run` starts a turn and loops through provider output and tool calls.
+- An `Interceptor` may continue, cancel, or pause work at defined orchestration boundaries.
+- `Engine::resume` continues paused generation without fabricating another user message.
+- Cancellation and provider errors are represented as typed `EngineError` values.
+
+## R2: Explicit cache-preserving state
+
+- `Engine<C, Mutable>` permits configuration and history edits.
+- `Engine::run` or `Engine::lock` transitions to `Engine<C, Locked>` and records the committed prefix.
+- A locked engine appends turns but cannot mutate that prefix through mutable-only APIs.
+- `Engine::unlock` explicitly abandons the lock before configuration or history changes.
+
+## R3: Tool declarations and execution
+
+- `#[tool_registry]` generates a schema and `Tool` implementation for methods marked `#[tool]`.
+- `#[description = "..."]` supplies argument descriptions in generated JSON Schema.
+- Generated code resolves its runtime and helper dependencies through `::agen`.
+- Invalid and duplicate marker attributes produce compile errors rather than panics.
+- Tools execute through `ToolServer` with typed context, errors, and output limits.
+
+## R4: Provider-neutral orchestration
+
+- `LlmClient` is the boundary between the engine and provider-specific transport.
+- Request/response schemes translate provider wire formats into shared request and event types.
+- Interceptors, tool execution, timeline collection, and pruning stay above the provider transport.
+- Provider-specific capabilities are optional features when they require additional policy or dependencies.
+
+## R5: Publication quality
+
+- crates.io metadata includes license, repository, documentation, README, categories, keywords, and MSRV.
+- The default feature set and each optional feature compile and test independently.
+- Macro expansion compiles in a downstream-style integration test without direct helper dependencies.
+- rustdoc builds without dependency documentation.
+- Package contents are explicitly bounded and exclude credentialed fixture-recording utilities.
+- `cargo package` and `cargo publish --dry-run` are run for `agen-macros` before `agen` because the main package depends on its companion package.