feat(diagnose): show per-check execution time in text output

[MsfPablo]

Closes #43

Summary

formatText rendered each check's status, severity and confidence but dropped the ExecutionTime that every check already records via SetExecutionTime. Users could not tell which check was slow from the text output, even though the data was already on CheckResult and present in the JSON/HTML/Markdown outputs.

Append the duration to each result's status line, human-readable via time.Duration.String():

  1. ✅ public_ip
     Status: passed | Severity: info | Confidence: 95% [1.2s]

Changes

• cmd/cli/commands/diagnose.go — formatText appends [%s] (the result's ExecutionTime) to the status line.
• cmd/cli/commands/diagnose_test.go — TestFormatTextIncludesPerCheckExecutionTime builds a report with per-result durations (1.2s, 350ms) and asserts both bracketed, human-readable forms appear.

Acceptance criteria (#43)

• Each check result in text output shows its execution time in brackets
• Time is formatted as human-readable (1.2s, 350ms)
• JSON/HTML/Markdown output already includes the field — no changes there
• A test verifies the time appears in text output

Verification

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