refactor: split tools.rs into focused submodules

[devfire]

Break the 1348-line tools.rs monolith into a tools/ directory with focused files:

• response.rs: ToolResponse struct
• options.rs: GrepOptions, FindOptions, OutputMode
• sinks.rs: grep-searcher Sink impls for each output mode
• common.rs: shared walker construction, filtering, error capture, drain_capped()
• grep.rs: grep() and the three mode functions (content/files/count)
• find.rs: find()
• cat.rs: cat()
• mod.rs: public re-exports and test helpers

Key improvements:

• Public API unchanged: mod.rs re-exports grep/find/cat and their option/response types, so server.rs requires zero changes
• Deduped: the identical ~20-line UTF-8 byte-cap drain loops are now common::drain_capped()
• Tests colocated: grep tests in grep.rs, find tests in find.rs, etc.
• No clippy warnings in tools/ (and fixed 4 pre-existing warnings in scope.rs)

Verification: cargo build clean, all 42 tests pass, clippy --all-targets 0 warnings.