Add completion auto-install and uninstall

[mnorth-stripe]

Summary

Add a colorized diff preview when stripe completion --install or --uninstall modifies shell config files. Users see the exact changes before approving.

• Colorized diff renderer (pkg/ansi/diff.go) with line numbers, context lines, multi-hunk support, and dark green/red true-color backgrounds
• Pure computeAddSentinel/computeRemoveSentinel functions (no I/O) for sentinel block manipulation
• Inline read → compute → diff → confirm → write flow — no wrapper abstractions
• Early shell name validation, Long/Example help text, quoted source paths
• 256-color line number foreground (bright green for adds, desaturated red for removes)
• True-color (24-bit) line backgrounds that span the full line including gutter

stripe completion --install (new file)

 ~/.zshrc (new file):
 
+    1 +# begin stripe-completion
+    2 +source "/Users/jane/.stripe/stripe-completion.zsh"
+    3 +# end stripe-completion
 
 Apply changes? [y/N]

stripe completion --uninstall (lines removed)

 ~/.zshrc:
 
     1  export PATH=/usr/local/bin:$PATH
     2  alias ll='ls -la'
-    3 -# begin stripe-completion
-    4 -source "/Users/jane/.stripe/stripe-completion.zsh"
-    5 -# end stripe-completion
 
 Apply changes? [y/N]

stripe completion --help

   Short: "Generate bash, zsh, and fish completion scripts",
+  Long:  "Generate shell completion scripts. Use --install to automatically
+          configure your shell profile, or run without flags to generate a
+          script file manually.",
+  Example: `  # Auto-install completions (detects your shell)
+  stripe completion --install
+
+  # Install for a specific shell
+  stripe completion --install --shell zsh
+
+  # Remove installed completions
+  stripe completion --uninstall
+
+  # Generate completion script to stdout
+  stripe completion --shell bash --write-to-stdout`,

Test plan

• go test ./pkg/ansi/... — 15 diff renderer tests (new file, add/remove/replace, multi-hunk, context capping, line alignment)
• go test ./pkg/cmd/... — pure function tests for computeAddSentinel/computeRemoveSentinel, readConfigFile, install/uninstall integration tests, confirmation accept/decline, manual remnant warnings
• golangci-lint passes
• Colors respect --color off, CLICOLOR=0, non-TTY piping (via ansi.Color(w))

[mnorth-stripe]

PR stack (3 of 3)

1. [Stacked] [completion --install] Add sentinel block management for shell config files #1485 — sentinel block management (base: fish-completion)
2. Add plugin completion protocol #1487 — plugin completion protocol (base: [Stacked] [completion --install] Add sentinel block management for shell config files #1485)
3. Add completion auto-install and uninstall #1488 ← you are here — completion auto-install (base: Add plugin completion protocol #1487)

[tomer-stripe]

Do we actually need a colorized diff? I'm unsure if it's meaningfully useful, especially as a default feature.

Also the pr title is confusing me, this isn't auto-installing or uninstalling?