feat: Merge template config into existing user config during setup install (#189)

* docs: Add config merging design spec for #114

Defines the merge-during-setup-install approach for keeping user
configs up to date with new template keys while preserving all
existing values, comments, and formatting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: Add implementation plan for config merging

TDD-based plan with 6 tasks: dependency setup, failing tests,
core merge implementation, integration test, WriteTemplateConfig
integration, and full verification.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: Merge template config into existing user config during setup install

Instead of skipping when a config file exists, WriteTemplateConfig() now
merges missing keys from the embedded template into the user's config
using YAML AST manipulation. Preserves all user values, comments, and
formatting. Only adds keys present in the template but absent in the
user's config.

Closes #114

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: Graceful error handling for config loading and setup commands

Replace panics in loadViperConfig with error returns so the app falls
back to defaults instead of crashing on malformed config files. Add
SilenceUsage to setup install/remove commands so runtime errors don't
dump the full usage text.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* add test cmd in readme

* update copy text

* refactor: Address review feedback on config merging

- Rename existing/template to dest/source for generic util naming
- Remove unnecessary code comments (Rule N references, obvious comments)
- Add AGENTS.md with dev guide and code style rules, symlink CLAUDE.md
- Add BenchmarkMergeYAML (~46μs/op on M4 Pro)
- Remove stale design spec

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* update `MergeYAML` to use from dry/utils

* update AGENTS.md

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

cmd/setup/setup.go

@@ -34,8 +34,9 @@ func NewSetupCommand() *cobra.Command {
 
 func NewInstallCommand() *cobra.Command {
 	return &cobra.Command{
-		Use:   "install",
-		Short: "Setup PMG config and aliases for package managers (npm, pnpm, pip, and more)",
+		Use:          "install",
+		Short:        "Setup PMG config and aliases for package managers (npm, pnpm, pip, and more)",
+		SilenceUsage: true,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			fmt.Print(ui.GeneratePMGBanner(version.Version, version.Commit))
 
@@ -63,8 +64,9 @@ func NewInstallCommand() *cobra.Command {
 
 func NewRemoveCommand() *cobra.Command {
 	cmd := &cobra.Command{
-		Use:   "remove",
-		Short: "Removes pmg aliases from the user's shell config file.",
+		Use:          "remove",
+		Short:        "Removes pmg aliases from the user's shell config file.",
+		SilenceUsage: true,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			fmt.Print(ui.GeneratePMGBanner(version.Version, version.Commit))