feat: add config show, test, and clear commands

[piekstra]

Summary

Adds three new config subcommands to complement the existing init command:

• cfl config show - Display current configuration with masked tokens and credential source indicators (env var name, config file, or not set)
• cfl config test - Test connectivity to Confluence with the current credentials, with clear error messages for auth failures
• cfl config clear - Remove the config file, with a note about active environment variables

Example output:

$ cfl config show
URL:        https://mycompany.atlassian.net/wiki  (source: config)
Email:      user@example.com  (source: ATLASSIAN_EMAIL)
API Token:  test********alue  (source: CFL_API_TOKEN)
Space:      DEV  (source: config)

Config file: ~/.config/cfl/config.yml

Fixes #94