fix(kit): stop expanding /learn course links to /docs paths on click

[sergiopaniego]

What does this PR do?

Fixes in-page navigation on course pages served at hf.co/learn/... when built with the current kit. On huggingface/agents-course, rebuilt on 2026-09-09 (first course rebuilt since the Svelte 5 migration), every sidebar and next-page click ends on a 404: huggingface/agents-course#736, huggingface/agents-course#737.

Since #794 the click listener in +layout.svelte rewrites the clicked anchor's pathname in the DOM through getHfDocFullPath ($lib/hfDocPaths.js), which also matched /learn/.... On a course page, a click on /learn/agents-course/unit1/tools becomes /docs/agents-course/main/en/unit1/tools before the Hub's own link handler reads it. The Hub then fetches /api/docs/agents-course/main/en/unit1/tools, which is a 404 (courses are only served under /api/learn/<course>/<page>), and falls back to a full navigation to the /docs URL, which is also a 404 for courses:

click /learn/agents-course/unit1/what-are-agents
  -> anchor rewritten to /docs/agents-course/main/en/unit1/what-are-agents
  -> fetch /api/docs/agents-course/main/en/unit1/what-are-agents   404
  -> location = /docs/agents-course/main/en/unit1/what-are-agents  404

Before #792 the same expansion lived inside the svelteKitCustomClient fork and only mutated SvelteKit's internal URL object, never the DOM anchor, so the Hub always saw the original /learn href.

Changes:

• getHfDocFullPath only expands /docs/... shorthand paths. /learn/... links are left untouched, so the Hub handles them via /api/learn/<course>/<page> as before.

The reroute hook never receives /learn URLs (SvelteKit treats URLs outside base as external before consulting it), so the removed branch was only reachable from the click listener.

Verified on the live agents-course page with fetch/beforeunload instrumented, blocking the anchor pathname write to emulate this change:

before:  fetch /api/docs/agents-course/main/en/unit1/what-are-agents 404, full navigation, Hub 404 page
after:   fetch /api/learn/agents-course/unit1/what-are-agents 200, lands on /learn/agents-course/unit1/what-are-agents

and the helper itself with node: /learn/agents-course/unit1/tools -> undefined, /docs/agents-course/unit1/tools -> /docs/agents-course/main/en/unit1/tools, /docs/trl/quickstart (other library) -> undefined. Prettier clean. kit/ has no JS test runner, so there is no unit test to add here without pulling in one.

Only agents-course is affected today. Every other course's bucket is still on a pre-#792 build, and each will break the same way on its first rebuild until this lands. After merging, agents-course needs a rebuild (re-run build_documentation.yml) to pick up the fixed kit, no agents-course-side change needed since the reusable workflow checks out doc-builder from main.

🤖 Generated with Claude Code

[sergiopaniego]

reported here: https://x.com/piyushJha__/status/2097967836027371782 :)