fix: git resource reliability (default branch + panic-safe errors)

[bmdavis419]

Summary

• detect repo default branch when branch is omitted
• improve branch omission UX in CLI add flow
• prevent internal match-wrapper panic messages from surfacing to users

Issues

• Closes Error: Unhandled exception: match err handler threw #153
• Closes btca fails when repository default branch is not "main" #159

Greptile Summary

Improved git resource handling by auto-detecting repository default branches when not explicitly specified, and prevented internal panic-wrapper error messages from surfacing to users.

• Auto-detects default branch using git ls-remote --symref when branch is omitted in CLI or API calls
• Falls back to 'main' if detection fails, maintaining backwards compatibility
• CLI now prompts with empty branch input and better descriptive text
• Internal error messages like "match err handler threw" now display as user-friendly messages
• Added proper git authentication support via BTCA_GIT_TOKEN environment variable
• Tests updated to reflect new error message format

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• Changes are well-structured with proper fallback handling, maintain backwards compatibility, and include test updates. The implementation is defensive with proper error handling and logging.
• No files require special attention

Important Files Changed

Filename	Overview
apps/cli/src/commands/add.ts	Improved UX for branch input with auto-detection support - branch now optional with empty default
apps/server/src/errors.ts	Added fallback wrapper message handling to prevent internal error messages from surfacing to users
apps/server/src/index.ts	Added git default branch auto-detection with proper authentication and fallback to 'main'

_{Last reviewed commit: a77d7a6}