Migrate to Viper for unified configuration management (bd-78)

- Add Viper dependency and create internal/config package
- Initialize Viper singleton with config file search paths
- Bind all global flags to Viper with proper precedence (flags > env > config > defaults)
- Replace manual os.Getenv() calls with config.GetString/GetBool/GetDuration
- Update CONFIG.md with comprehensive Viper documentation
- Add comprehensive tests for config precedence and env binding
- Walk up parent directories to discover .beads/config.yaml from subdirectories
- Add env key replacer for hyphenated keys (BD_NO_DAEMON -> no-daemon)
- Remove deprecated prefer-global-daemon setting
- Move Viper config apply before early-return to support version/init/help commands

Hybrid architecture maintains separation:
- Viper: User-specific tool preferences (--json, --no-daemon, etc.)
- bd config: Team-shared project data (Jira URLs, Linear tokens, etc.)

All tests passing. Closes bd-78, bd-79, bd-80, bd-81, bd-82, bd-83.

Amp-Thread-ID: https://ampcode.com/threads/T-0d0f8c1d-b877-4fa9-8477-b6fea63fb664
Co-authored-by: Amp <amp@ampcode.com>

go.sum

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