Release 0.17.0

Added

• Step Metadata: Four declarative fields on BaseStep: match_modules (glob patterns for selective execution), ignore_errors (fault-tolerant steps), pure (safe for validate() dry-run), timeout_ms (per-step timeout).
• YAML Pipeline Configuration: register_step_type(), unregister_step_type(), registered_step_types(), build_strategy_from_config() — configure pipeline steps via apcore.yaml at startup.
• PipelineContext fields: dry_run, version_hint, executed_middlewares for pipeline-aware execution.
• StepTrace: skip_reason field for understanding why steps were skipped ("no_match", "dry_run", "error_ignored").

Changed

• Step order: middleware_before now runs BEFORE input_validation (was after). Middleware input transforms are now validated by the schema check.
• Executor delegation: call(), call_async(), validate(), and stream() fully delegate to PipelineEngine.run(). Removed ~300 lines of duplicated inline step code.
• Renamed: safety_check step → call_chain_guard (accurately describes call-chain depth/cycle/repeat checking).
• Renamed: BuiltinSafetyCheck class → BuiltinCallChainGuard.

Fixed

• Middleware input transforms were never re-validated against the schema (now validated after middleware runs).
• validate() was hardcoded to 7 inline checks; now uses dry_run=True pipeline mode — user-added pure=True steps automatically participate.

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