feat(config): add git.author and git.no-gpg-sign options (fixes #600)

Adds configuration options for beads git commits:
- git.author: Override commit author (e.g., "beads-bot <beads@example.com>")
- git.no-gpg-sign: Disable GPG signing for beads commits

This is useful for users with Touch ID commit signing (like Secretive)
who get prompted for every beads auto-commit.

Config example:
```yaml
git:
  author: "beads-bot <beads@example.com>"
  no-gpg-sign: true
```

Environment variables: BD_GIT_AUTHOR, BD_GIT_NO_GPG_SIGN

Closes #600

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/config/config.go

@@ -112,6 +112,10 @@ func Initialize() error {
 	// Create command defaults
 	v.SetDefault("create.require-description", false)
 
+	// Git configuration defaults (GH#600)
+	v.SetDefault("git.author", "")        // Override commit author (e.g., "beads-bot <beads@example.com>")
+	v.SetDefault("git.no-gpg-sign", false) // Disable GPG signing for beads commits
+
 	// Read config file if it was found
 	if configFileSet {
 		if err := v.ReadInConfig(); err != nil {