Phase 1: terminal output overhaul - result summary, severity table, --quiet/--no-color

[advaitpatel]

Summary

Reworks DockSec's terminal output into a clean, consistent result summary and fixes the
long-standing progress-bar and exit-code issues. Scanning logic is unchanged.

Changes

New output layer (docksec/output.py)

A single module owns the look of everything printed to the terminal: one shared Rich
console, section/status helpers, a box-drawing severity table, the score line, a
"Quick take" action block, the report list, and a suggested next command. No emoji;
structure comes from color and box-drawing borders only.

Result summary

Every scan now ends with: a severity table (Critical/High/Medium/Low), the security
score with a color-coded rating, a Quick take highlighting the most important findings,
the generated reports, and a suggested next command.

Fixed

• Report progress no longer prints each step twice / out of order (reports generate
  silently; the CLI renders one report line from the returned paths).
• A failed scan (e.g. image not found) now exits non-zero instead of ending with a
  misleading "Analysis complete!".

Added

• --quiet - reduce output to warnings, errors, and the result summary.
• --no-color - disable colored output (also honors the NO_COLOR env var).

Testing

• ruff check . passes.
• pytest: 102 passed (new test_output.py; updated scanner/CLI tests).
• Manual runs verified for scan-only, --quiet, --no-color (0 ANSI escapes), and
  --compose.

Notes

• Scope is deliberately contained to CLI + summary/report output. A few verbose
  error-path troubleshooting prints in docker_scanner still use print(); routing
  those is a low-priority follow-up.
• Format selection and --severity/--fail-on/--json-to-stdout remain Phase 2.