Make the Mint broken-link check required

[innovativedesign-lab]

Close: #185

Here's what was done and why each decision was made:

What was built

check-broken-links.mjs
— a standalone Node.js checker (no mint CLI required) with three check categories:

Internal routes — resolves every link against the set of real .md/.mdx files, accounts for redirect sources in docs.json, and explicitly checks /assets/* paths
Heading anchors — slugifies headings the same way Mintlify does and verifies #fragment links point at real headings or id= attributes
External URLs — HEAD→GET fallback, up to 3 retries with exponential back-off + jitter, classifies each failure as PERMANENT (4xx, ENOTFOUND, ECONNREFUSED) or RETRYABLE (5xx, 429, 503, timeout); exits 1 only on permanent failures
--no-external flag — runs instantly with zero network calls, used by the PR step.

Two new npm scripts in package.json:

check:broken-links # full check (internal + external)
check:broken-links:internal # internal only (fast, offline-safe)
New broken-links job in snippets.yml:

No needs: dependency — runs in parallel with the other jobs
No continue-on-error — it's a required check that fails the PR
Internal check on every PR and push; external check only on push (keeps PR feedback fast)

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[truthixify]

The new required job fails on eight internal links, so the branch does not leave the gate green. More importantly, the external checker performs unrestricted fetch(..., redirect: follow) for repository-controlled URLs without rejecting private, loopback, link-local, mapped IPv6, or redirect destinations. On the post-merge push job this is a CI SSRF path. Fix or explicitly model the eight shipped targets, resolve and pin only globally routable addresses, follow redirects manually while revalidating every hop, and add private-address/DNS-rebinding tests.