[dmt] add statistic after linter run

[ldmonster]

What

Add an end-of-lint statistics summary that is printed after the regular lint
results. The summary is rendered as a single framed block styled to match the
deckhouse-cli mirror pull/push summaries, so the two tools feel like one family.

Running dmt lint now finishes with a compact overview of the run:

╔══ Lint summary ═══════════════════════════════════════
║ Modules:    3
║
║ Critical:   0
║ Errors:     5
║ Warnings:   12
║ Ignored:    2
║
║ By linter:
║   templates                      12
║   openapi                        3
║   no-cyrillic                    2
║
║ Total:      19 findings
║
║ Lint failed; 5 problem(s) must be fixed.
║ Elapsed: 1.2s
╚═══════════════════════════════════════════════════════

Why

Until now the output was a flat list of findings with no roll-up: on a large run
you had to scroll and count by hand to answer basic questions — how many modules
were scanned, how the findings break down by severity, which linters fired most,
and how long the run took. The summary puts that at the bottom of every run and
gives CI logs a single, scannable verdict line.

Details

• internal/manager/statistics.go — new file with the collector and renderer:
  • collectStatistics() tallies all collected findings by severity
    (critical / errors / warnings / ignored) and by linter. Counts always cover
    every finding regardless of the --hide-warnings / --show-ignored display
    flags, so the summary reflects the full picture.
  • renderStatistics() formats the framed block. The title and state line adapt
    to the outcome:
    • errors/critical present → title Lint failed, red Lint failed; N problem(s) must be fixed.
    • warnings only → Completed with N warning(s).
    • clean run → green No problems found. (per-linter breakdown omitted)
  • The per-linter breakdown is sorted most-findings-first, ties broken by name.
  • Colours are semantic (red = failure, yellow = attention, green = clean, dim =
    zero/secondary) and go through fatih/color, which disables ANSI when stdout
    is not a TTY or NO_COLOR is set — so escape codes never leak into pipes or
    files. Colours are applied after width padding so they don't break columns.
• internal/manager/manager.go — Manager gains a startedAt timestamp set
  in NewManager; the summary reports wall-clock elapsed since it.
• cmd/dmt/main.go — mng.PrintStatistics() is called right after
  mng.PrintResult() in runLint.