docs: CLI improvements documentation

[himanshu231204]

Updates documentation for the CLI improvements:

Changes

• README.md: Added new features to Features table, updated CLI Commands section with all new commands
• docs/06_cli_spec.md: Full rewrite with all 9 commands, global flags, config auto-discovery
• docs/cli.md: Comprehensive CLI reference with all new commands and flags
• docs/quickstart.md: Added tips for interactive wizard, dry-run, and metric override
• CHANGELOG.md: Added CLI Improvements section to [Unreleased]

New Documentation

• Validate command reference
• Delete command reference
• Dry-run mode explanation
• Shell completion installation instructions
• Config auto-discovery rules
• Exit codes reference (0-5)

Related

• PR feat: comprehensive CLI UX improvements #24: CLI improvements code
• Closes Fix for Statement has no effect #19, fix: comprehensive bug hunt — 25 fixes across Critical, High, Medium, Low severity #21, docs: add provider guides for all LLM, retriever, and embedder providers #22