refactor: reorganize into idiomatic Go project layout

[timvw]

Summary

• Restructure flat package main into idiomatic Go CLI layout: cmd/ package for cobra commands, internal/color/ and internal/fuzzy/ for reusable utilities, and a slim main.go entry point
• Extract color utilities and fuzzy matching into dedicated internal packages with exported APIs
• Update goreleaser ldflags from main.version to cmd.Version

What changed

wt/
├── main.go                    # 6 lines: just calls cmd.Execute()
├── cmd/                       # all cobra commands + CLI logic
│   ├── root.go                # rootCmd, Execute(), init/flag setup
│   ├── checkout.go, create.go, ...
│   └── *_test.go
├── internal/
│   ├── color/                 # ANSI color support (Colorize, IsEnabled)
│   └── fuzzy/                 # fuzzy matching for interactive selectors
├── e2e/                       # (unchanged)
└── docs/                      # (unchanged)

Test plan

• go build ./... succeeds
• go test -short ./... — all tests pass
• Version ldflags injection verified: go build -ldflags "-X github.com/timvw/wt/cmd.Version=1.2.3"
• Binary runs correctly (wt version, wt --help)
• CI pipeline passes

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 29.14%. Comparing base (5be4d4e) to head (a5bfe3b).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/root.go	70.11%	24 Missing and 2 partials ⚠️
cmd/pr.go	0.00%	2 Missing ⚠️
cmd/version.go	0.00%	2 Missing ⚠️
cmd/checkout.go	0.00%	1 Missing ⚠️
cmd/remove.go	0.00%	1 Missing ⚠️
cmd/status.go	92.85%	1 Missing ⚠️
main.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
+ Coverage   29.12%   29.14%   +0.01%     
==========================================
  Files          26       27       +1     
  Lines        2712     2714       +2     
==========================================
+ Hits          790      791       +1     
  Misses       1843     1843              
- Partials       79       80       +1     

Files with missing lines	Coverage Δ
cmd/cleanup.go	31.09% <ø> (ø)
cmd/config.go	68.06% <ø> (ø)
cmd/config_cmd.go	58.49% <ø> (ø)
cmd/create.go	0.00% <ø> (ø)
cmd/default.go	41.66% <ø> (ø)
cmd/examples.go	67.74% <ø> (ø)
cmd/hooks.go	62.71% <ø> (ø)
cmd/info.go	0.00% <ø> (ø)
cmd/init.go	42.18% <ø> (ø)
cmd/list.go	0.00% <ø> (ø)
... and 16 more

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