feat(diagnose): add --no-color flag for CI/scripting

[MsfPablo]

Closes #42

Summary

formatText and formatComparisonText prefix the report with a 📊 header and each result with an emoji status glyph (✅/❌/⚠️). That breaks CI logs, log files, and pipes that expect plain text — the emoji render as escapes or mojibake in many log viewers and terminals without UTF-8.

Add a --no-color flag (default false) that, when set, swaps the status emoji for plain bracketed markers and drops the header glyph, so the text report is safe to grep, store, and pipe.

$ proxyctl diagnose --url https://example.com --no-color
Diagnosis Results
────────────────────────────────────────────────

1. [PASS] public_ip
   Status: passed | Severity: info | Confidence: 90%
   Public IP resolved via proxy.

2. [FAIL] dns_leak
   Status: failed | Severity: critical | Confidence: 80%
   DNS bypassed the proxy.

Changes

• cmd/cli/commands/diagnose.go
  • --no-color bool flag registered on diagnoseCmd (default false).
  • statusMarker(result) helper: returns [PASS]/[FAIL]/[ERROR] when --no-color is set, otherwise the existing emoji.
  • textHeader(label) helper: returns the bare label when --no-color is set, otherwise 📊 <label>.
  • formatText and formatComparisonText use the helpers.
• cmd/cli/commands/diagnose_test.go
  • TestFormatTextOmitsEmojiWhenNoColor: with noColor=true, asserts no 📊/✅/❌/⚠️ remain, [PASS]/[FAIL] and the header label appear, and that formatMarkdown is unaffected (still uses emoji).

Acceptance criteria (#42)

• --no-color flag exists and defaults to false
• When set, text output contains no emoji
• JSON/HTML/Markdown output is unaffected
• A test verifies the flag behavior

Verification

go build ./...
go test ./cmd/cli/commands/
gofmt -l cmd/cli/commands/
go vet ./cmd/cli/commands/

[francomano]

Richiesta di modifica: --no-color promette di disabilitare le emoji per CI/log/pipeline, ma viene applicato soltanto al report formattato. Rimangono emoji nel banner, nelle righe "Running diagnosis"/"Via proxy", nei messaggi di errore e nel messaggio di salvataggio. Centralizza i marker/heading o rendi condizionali anche questi percorsi, quindi aggiungi un test end-to-end del comando che verifichi l’assenza di emoji nell’intero stdout.

[francomano]

English version of my change request: --no-color promises to disable emoji for CI, logs, and pipelines, but it currently only affects the formatted report. Emoji remain in the banner, the "Running diagnosis" / "Via proxy" lines, error messages, and the save confirmation. Please centralize these markers/headings or make those paths conditional as well, then add an end-to-end command test confirming that the complete stdout contains no emoji.