fix(hooks): rewrite producer stage of producer | cat pipelines

[javython999]

A bare cat (no flags, no files) is byte-transparent: it copies stdin to stdout unchanged. producer | cat and producer are therefore equivalent on stdout, so the producer segment can be rewritten the same way a standalone command would be, without risking a change to what a downstream consumer reads.

Previously the hook left pipelines entirely raw except for an allowlisted final-stage rewrite (grep/rg), so git status | cat never got the rtk rewrite even though nothing consumes the pipe's output structurally.

The exception is scoped to a single stdout pipe with no further pipe stages, so pipelines with real intermediate processing (a | b | cat) keep the existing raw-producer contract.

Fixes #3722

Summary

• Rewrite the producer stage of a producer | cat pipeline (e.g. git status | cat -> rtk git status | cat) since a bare cat sink is byte-transparent
• Added is_transparent_cat_sink / rewrite_producer_before_transparent_sink in src/discover/registry.rs, wired as a fallback when the existing final-stage rewrite finds nothing
• Scoped narrowly: only a single stdout pipe with no further pipe stages qualifies; a | b | cat, cat -n, and |& stderr pipes are unaffected

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• Added 5 new unit tests in src/discover/registry.rs covering the happy path, && continuation after the pipe, cat -n (non-transparent), a real intermediate stage before cat, and the stderr-pipe case
• Manual testing: echo '{"tool_name":"Bash","tool_input":{"command":"git status | cat"}}' | rtk hook claude now rewrites to rtk git status | cat

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