feat: add version-aware Silkworm development skill

[egorsmkv]

Closes #21

Add skills/develop-silkworm: SKILL.md, agent metadata, five focused references and a complete crawler/test template grounded in the current 0.2.1 code and documentation. Cover spiders, CSS/XPath pagination with origin checks, custom middleware/pipelines, concurrency, HTTP retries, resource limits, structured logging, performance measurement and deterministic regressions.

Require agents to resolve the installed version/source/features before using recipes and recheck signatures after upgrades. Document retry exhaustion, transport-error limits, backlog overload, truncation and pipeline shutdown semantics.

Add a locked example checker to existing CI and link the skill from README. Tests exercise local HTTP, header injection, enriched JSONL flush, extraction, pagination boundaries, final-status rejection and retry exhaustion.

Validation passed:

• Skill schema validation, cargo fmt, asset rustfmt and git diff --check.
• cargo check --all-targets.
• cargo test --all: 126 framework tests passed.
• Bundled example: default (3 tests), XPath (4 tests), no-default (3 tests).
• scripts/check_docs.py: 25 Rust snippets compiled in three feature configurations and local quickstart tests passed.
• Independent skill-use pass confirmed the workflow and operational limits.

Local verification used Rust 1.99 and disabled debug symbols to fit available disk space. Existing CI retains its MSRV and feature checks. No library API/runtime changes.