feat(nix): add Nix ecosystem command handling

[jpds]

Summary

Takes #1046 and rebases it on the latest develop - filter nix command output (build, develop, flake, shell, env) to cut token usage: shorten store path hashes, batch download/copy progress into summaries, collapse store listings, strip evaluation traces, and keep errors and warnings intact.

Changes done on top of the previous branch:

• Adapted to current APIs:
  • lazy_static replaced with std::sync::LazyLock
  • RtkRule entries built from ..RtkRule::DEFAULT (new pipeline_final_safe field)
  • Four nix subcommands added to the passthrough classification list checked by test_every_subcommand_is_classified
• Error and warning lines were previously pushed verbatim (keeping full 32-char store hashes) - they now go through compress_store_paths like every other kept line
• Fetch-plan headers ("these N paths will be fetched (... MiB ...)"), which real nix always prints with size annotations, didn't match PLAN_HEADER_RE: the header was dropped entirely and its paths leaked through uncompressed. The pattern now covers them, and the indented paths are counted into the collapsed list summary
• Removed unused PHASE_RE; simplified ERROR_RE; merged duplicated run()/run_legacy() into one run_tool()
• Replaced the five-parameter flush_counters() function with a module-local Counters struct

Test plan

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

Important: All PRs must target the develop branch (not master).
See CONTRIBUTING.md for details.

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