feat: add --list and --dry-run flags to gomu run

[xen0l]

Summary

Adds two discovery-only flags to gomu run, implementing #90. Both stop short of the execution phase, so they are fast and side-effect-free.

-l, --list

Prints the catalog of supported mutators with one-line descriptions and exits — no path, no AST walk, no mutation.

$ gomu run --list
Supported mutators (17):
  arithmetic          Replace arithmetic operators (+, -, *, /, %)
  conditional         Replace comparison operators (==, !=, <, <=, >, >=)
  ...

--dry-run

Runs the real file-selection and mutant-generation pipeline and prints which mutations would run per file, but executes nothing (no overlay build, no go test, no report, no CI gate). It honors --incremental, so it reflects exactly what a real run would process; --incremental=false previews every file.

$ gomu run --dry-run ./...
Dry run: analyzing 2 file(s) (no mutations will be executed)

pkg/foo/foo.go (3 mutants)
  L12:7  arithmetic_binary  Replace + with -
  ...

Total: 3 mutant(s) across 1 file(s)

Implementation

Respects the existing cmd/gomu → pkg/gomu → internal/mutation layering.

• internal/mutation: adds Description() to the Mutator interface (implemented by every mutator) and an exported SupportedMutators() that returns the registry without constructing an analyzer. No new mutator structs, so registry.go and its generator are untouched (go generate is a no-op).
• pkg/gomu: adds public MutatorInfo + SupportedMutators(); adds a DryRun option; Run() branches to an io.Writer-based dry-run formatter right after file discovery, before any execution or reporting.
• cmd/gomu: registers --list/-l and --dry-run on the run command; --list is handled first via listMutators using text/tabwriter for alignment.

Tests

• every mutator reports a unique, non-empty Name() / Description()
• dry-run formatter output, the SupportedMutators() mapping, and an end-to-end Run(DryRun) test asserting no report or history file is produced
• listMutators output

Verification

go build ./..., go vet ./..., go test ./... pass; go generate ./internal/mutation leaves registry.go unchanged.

[sivchari]

It's better to print relative path, not absolute path.

[xen0l]

I chose working-directory relative paths. The issue fixed in e8a660c