fix(hook): add output transparency section to RTK instructions

[guyoron1]

Summary

When RTK is installed as a silent PreToolUse auto-rewrite hook, safety-tuned LLMs can flag RTK's filtered output as tampered because the output shape differs from what the original command produces (e.g., grep results grouped by file, diffs compacted, lists truncated with +N more).

This PR adds an "Output Transparency" section to the RTK_INSTRUCTIONS constant (injected into CLAUDE.md by rtk init). The section explicitly tells the LLM that:

• RTK intentionally compresses and restructures output
• Output may look different from raw commands — this is expected and correct
• rtk proxy <cmd> is available for exact unfiltered output

This converts "unexplained output mismatch" into "expected behavior from a known tool," addressing the most common trigger for tamper heuristics.

Changes

• src/hooks/init.rs: Add 4-line "Output Transparency" section to RTK_INSTRUCTIONS

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test --all passes
• Existing test_init_contains_key_commands and test_init_has_version_marker tests pass
• No changes to filter pipeline, hook processor, or output format

Fixes #2445