refactor: extract duplicated status/report logic into shared utilities

[devin-ai-integration]

Summary

Five patterns were duplicated across formatter.ts, report.ts, scanner.ts, cli.ts and github.ts. They now live in one place, so adding a status or a summary counter is a single-file change instead of four. Output is unchanged (verified byte-for-byte against the pre-refactor code, see below).

New src/link-status.ts owns everything keyed by LinkStatus:

• STATUS_DISPLAY — replaces the parallel statusEmoji() (report) / statusIcon() (formatter) switch statements, which drifted apart for insecure-http. Stores { emoji, symbol, color } with color as a name ('green' | ...) so the module stays chalk-free; formatter.ts maps names to chalk via a local COLORS record.
• SUMMARY_ROWS — the Markdown summary table and the console counters line were two hand-maintained lists of the same 7 counters, in the same order, with different labels. Now one list of { key, emoji, label, shortLabel, color }:

  // report.ts
  for (const row of SUMMARY_ROWS) lines.push(`| ${row.emoji} ${row.label} | ${summary[row.key]} |`);
  // formatter.ts
  SUMMARY_ROWS.map((row) => COLORS[row.color](`${summary[row.key]} ${row.shortLabel}`)).join('  ')

• isFailureStatus / isHealthy / hasIssue — the filter predicates previously inlined in both reporters (r.status === 'broken' || r.status === 'malformed' || r.status === 'timeout', etc.). They accept a structural LinkFacets, so no import cycle with scanner.ts.
• STATUS_SUMMARY_KEY — summarize() in scanner.ts had one if per status; it now looks up the counter key. insecure-http maps to null (it is counted via the isHttp flag), preserving current behaviour.
• shouldFailScan(counts, thresholds) — the identical 3-clause exit-code expression in cli.ts and github.ts. Both call sites pass config, which already satisfies FailureThresholds.

ScanSummary now extends StatusCounts instead of re-declaring the 7 counters.

Also deduplicated:

• URLDN_SHORTENER_URL was defined twice → src/constants.ts.
• error instanceof Error ? error.message : String(error) appeared 4× → toErrorMessage() in utils.ts; isValidUrl/isInsecureHttp shared a try/catch new URL() → parseUrl().
• printConsoleReport repeated the same "heading, loop, per-entry line, dimmed detail lines" block 5× → one printSection({ heading, icon, color, results, limit?, suffix?, details?, footer? }) helper.
• checkUrl built the same CheckResult literal 5× → a local result() closure filling url/isHttp/durationMs/redirectChain, plus classifyStatusCode() for the 2xx/3xx/else branching.
• INLINE_LINK_RE and IMAGE_MARKDOWN_RE were the same pattern modulo a leading ! → composed from shared LINK_LABEL/LINK_TARGET sources.
• upsertPullRequestComment spread context.repo instead of repeating owner:/repo: three times.

Verification

Beyond lint, format:check, typecheck and the suite (new test/link-status.test.ts, plus tests for parseUrl/toErrorMessage), I ran a throwaway parity test that imported the pre-refactor src/ from git archive HEAD alongside the new one and asserted identical output for: the console report (over all 48 status/flag combinations, including the ">20 ok links" truncation branch), every verbose line, the Markdown report (issues and no-issues paths), checkUrl across status codes 100..500 with and without redirects, extractLinksFromMarkdown, and scan summaries. All matched exactly; the scaffold was deleted and is not part of this PR.

Link to Devin session: https://app.devin.ai/sessions/5084b99d963a4a8db00d7c0bcbce1c74
Requested by: @lahcenassmira

[devin-ai-integration]

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