feat(hook): extend Gemini BeforeTool hook to intercept read_file tool…

[Ross-cripto]

… #796

• Route read_file alongside run_shell_command in run_gemini()
• Pre-apply start_line/end_line slice before filtering, then override both in the response to prevent Gemini's merge from re-applying original (now wrong) line numbers to the filtered file
• Write filtered content to ~/.local/share/rtk/filtered/{ts}-{stem}.rtk (timestamp-first so lexicographic sort in rotation is chronological)
• Rotate filtered dir to keep last 20 files, mirroring tee.rs pattern
• Fallback to allow for unreadable/binary files, slices <500 chars, and slices with <10% savings

Closes #796

Summary

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• Manual testing: rtk <command> output inspected

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