feat: nvi-demux init - interactive configuration generator

[georgemarselis-nvi]

nvi-demux init generates a configuration.json by walking the operator
through a series of prompts. Output is a valid configuration file ready for
nvi-demux configure --config configuration.json (#206).

Prompts:

• sequencer raw data directory
• demultiplex output directory
• for_transfer directory
• NIRD host alias (from ~/.ssh/config)
• IRIDA base URL
• IRIDA OAuth2 client ID
• Teams webhook URL
• notification email recipients (comma-separated)
• Bitwarden item UUIDs (or skip if not yet configured)

Behaviour:

• validates each input before proceeding (path exists, URL reachable, alias
  present in ~/.ssh/config)
• warns but does not abort on unreachable optional services (Teams, Bitwarden)
• writes configuration.json to current directory by default
• --output flag to specify destination path
• --dry-run flag to print without writing

Depends on: #163 (PyPI packaging), #206 (installation script)