feat(pipe): accept built-in and trusted custom TOML filters in pipe -f

[rNoz]

Summary

• rtk pipe -f <name> only resolved the hard-coded Rust filter names, so trusted project/global TOML filters (and built-in TOML filters) were rejected with Unknown filter '<name>', even though rtk rewrite and the hook already apply them.
• Now, when no Rust filter matches, the name is resolved against the TOML filter registry. Only built-in and trusted custom filters are loaded (trust gating is enforced by the registry), so untrusted filters are still rejected.
• A Filter enum dispatches Rust vs TOML through the same never_worse guard, preserving safe fallback behavior.
• The Unknown filter message now lists available TOML filter names in addition to the built-in Rust names.
• Solving Refs Custom TOML filters (global or trusted project-local) are never applied by the hook #2179 (custom TOML filters never applied). The hook/rewrite path is already fixed on develop; this closes the remaining rtk pipe -f loose end.

Design

• src/core/toml_filter.rs: two new public functions over the existing lazy REGISTRY:
  • find_filter_by_name(name) -> Option<&'static CompiledFilter>: name lookup with project > global > built-in precedence (same load order as find_matching_filter). Untrusted custom filters are never in the registry, so they are never returned.
  • loaded_filter_names() -> Vec<&'static str>: sorted, de-duped, for error messages.
• src/cmds/system/pipe_cmd.rs:
  • enum Filter { Rust(fn(&str)->String), Toml(&'static CompiledFilter) }.
  • run() tries the built-in Rust resolver first, then the TOML registry by name, else errors with a dynamic message.
  • apply_filter(&Filter, input) dispatches and keeps the panic-catch fallback; never_worse is still applied to the result.

Trust model (explicit)

• Built-in TOML filters (src/filters/*.toml) are always trusted and usable via pipe -f.
• Project-local (.rtk/filters.toml) and user-global (~/.config/rtk/filters.toml) custom filters require rtk trust (or the CI-gated RTK_TRUST_PROJECT_FILTERS=1). Untrusted custom filters are rejected.
• pipe -f <name> matches by filter name only; match_command is irrelevant here (pipe has no command string).

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test --all: pass.
• New tests/pipe_custom_filter_test.rs:
  • trusted_custom_filter_is_applied: trusted .rtk/filters.toml filter runs via pipe -f (noise lines stripped).
  • untrusted_custom_filter_is_rejected: same filter, no trust, exits non-zero with Unknown filter.
  • builtin_toml_filter_works_without_trust: built-in make filter usable via pipe -f.
  • rtk_no_toml_disables_toml_filters_in_pipe: with RTK_NO_TOML=1, a TOML filter name is rejected (Unknown filter).
  • builtin_rust_filter_still_works: grep still works (no regression).
• New unit tests in pipe_cmd.rs: unknown_filter_message lists built-ins; find_filter_by_name("make") resolves; unknown name returns None.

e2e validation

So far so good, but I will do a final round of e2e checks tomorrow, and set ready for review.

[CLAassistant]

All committers have signed the CLA.

[rNoz]

Closed as forgot one CONTRIBUTING statement (too used to always start with rnoz/ prefix), so favouring the proper PR here: #2999