feat(cli): colour deco check output

[tlgimenes]

Colours deco check's report, as in the demo film. No new dependencies; a 30-line ANSI helper (packages/blocks/src/v8/cli/color.ts).

When: on only when stdout and stderr are both terminals, or with --color (any command); --no-color turns it off, the last one wins. No environment variables are read. The setting rides on the Reporter, so piped output and tests stay plain, and exit codes don't change.

Before (any output):

2 saved blocks checked: 0 errors, 0 warnings

After, on a terminal (cat -v):

^[[32m✓^[[39m 2 saved blocks checked: ^[[32m0 errors, 0 warnings^[[39m

• clean: green ✓, counts green
• warnings only: yellow ⚠, warning count yellow (exit 0)
• errors: red ✗, error count red (exit 1)
• problem lines: file bold, path dim, an error's message red, the word warning yellow. Strip the escapes and it's the plain text again.

The mark leads rather than trails: it's where the eye lands first and where most CLIs (vitest, gh) put it, and it stays in the same column whatever the counts. deco schema gets the same yellow warning and red "not written" line.

Tests: --color summaries (clean/warnings/errors), problem-line escapes, strip-to-plain, --color/--no-color parsing on every command, --color=x rejected. Existing plain-text tests unchanged.

🤖 Generated with Claude Code

https://claude.ai/code/session_01WNwbSEePYNcY5YCgqZURig

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Summary by cubic

Colours deco check and deco schema output so reports read well on a terminal. Piped output and tests stay plain text, and exit codes are unchanged.

• The check summary leads with a green ✓ when clean, yellow ⚠ when only warnings, red ✗ when there are errors; problem lines are bold files, dim paths, red error messages, and yellow warning words.
• Every command takes --color or --no-color, the last one winning; without either, colour turns on only when stdout and stderr are both terminals.

^{Written for commit 98feda7. Summary will update on new commits.}