fix(api): standardize Fetchkit casing

[chaliy]

What

Standardize the product spelling to Fetchkit across docs, specs, comments, package descriptions, examples, CLI help text, and the default User-Agent.

Why

The proper product name is Fetchkit, not FetchKit. Public surfaces should reflect that consistently.

How

• Renamed the canonical Python binding class to FetchkitTool.
• Kept FetchKitTool(...) as a deprecated compatibility constructor that emits DeprecationWarning.
• Added a deprecated Rust alias for the old wrapper type.
• Added a spec rule that old FetchKit identifiers are compatibility-only.

Risk

• Low / Medium
• Python callers using FetchKitTool still construct a tool, but FetchKitTool is now a deprecated function shim rather than the canonical class object.

Checklist

• Unit tests are passed
• Smoke tests are passed
• Documenation is updated
• Specs are up to date and not in conflict
• Security review performed: no new URL/network behavior beyond User-Agent spelling; compatibility shim only emits a Python warning.

Validation:

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps
• cargo build --workspace --exclude fetchkit-python --release
• cargo run -p fetchkit-cli -- --help
• cargo run -p fetchkit-cli -- mcp --help
• cargo run -p fetchkit --example fetch_urls