feat(mix): add Elixir Mix command filter

[martinstannard]

Summary

• Adds a token-optimized filter for Elixir's mix build tool, the primary toolchain for Elixir/Phoenix projects
• Supports 6 subcommands: mix test, mix compile, mix credo, mix phx.routes, mix ash.codegen, mix ash_postgres.generate_migrations
• Includes discover integration with per-subcommand savings estimates (70-90%)

What it does

mix test

• Compact output: passing tests → single summary line; only shows failures with file/line/description
• Preserves compilation errors before test output (common in Elixir test runs)
• Handles both ExUnit and doctest output formats
• TTY-aware: respects verbose flag for full passthrough

mix compile

• Warnings grouped by file, error details preserved
• Empty output when clean (no noise)
• Preserves "Compiling N files" progress line

mix credo

• Keeps high/medium priority issues with file/line/column/message
• Hides low-priority (refactoring) suggestions to cut noise
• Verbose passthrough preserves all priorities

mix phx.routes

• Full passthrough (routes are already compact and essential for LLM context)

mix ash.codegen / mix ash_postgres.generate_migrations

• Strips verbose JSON snapshot output from Ash codegen
• Preserves migration SQL blocks and status summaries
• Shows "check ok" or "check pending" status clearly

General

• All commands fall back to raw output on filter failure
• Non-zero exits still print filtered stdout (credo/dialyzer write reports to stdout on failure)
• Tracking integration for rtk gain statistics
• Discover registry: Elixir category with per-subcommand token savings

Token Savings

Subcommand	Est. Savings
mix test	90%
mix compile	85%
mix credo	80%
mix phx.routes	70%
mix ash.codegen	88%
mix ash_postgres.generate_migrations	88%

Changes

File	+
src/mix_cmd.rs	+946 (new module, 24 tests)
src/main.rs	+11 (Commands::Mix enum + routing)
src/discover/registry.rs	+1 (Elixir category)
src/discover/rules.rs	+17 (mix rule entry)

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• 24 unit tests covering: all-pass, failures, multiple failures, compilation errors, verbose passthrough, compile warnings/errors/clean, credo priority filtering, codegen JSON stripping, migration SQL preservation, routes passthrough, help text
• Manual testing: rtk mix test, rtk mix credo, rtk mix compile, rtk mix phx.routes on real Phoenix project

[CLAassistant]

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