fix(claude-config): fetch-docs index_link resolves a slug to a nested page with the same basename

[kyle-sexton]

Problem

plugins/claude-config/scripts/fetch-docs.sh resolves a bare doc slug to the first link in the docs index whose URL ends in /<slug>.md (index_link, line 212 on origin/main). A nested page with the same basename wins when the index lists it first.

The live index at https://code.claude.com/docs/llms.txt lists https://code.claude.com/docs/en/plugins/cli-reference.md (line 155) before https://code.claude.com/docs/en/cli-reference.md (line 267). Fetching slug cli-reference therefore records the plugins page as read. doc-citations.tsv cites cli-reference with the span --debug-file, which exists only on the top-level page, so check-doc-citations.sh reports a false "span missing" failure against a page it graded as read. --discover also marks the real page not-in-index, for the same reason.

hooks, permissions and skills have agent-sdk/<slug>.md siblings and resolve correctly today only because the index lists the top-level page first.

The old direct URL form (/docs/en/$slug.md) did not have this collision; it came in with #5440.

Evidence

• git show origin/main:plugins/claude-config/scripts/fetch-docs.sh: index_link matches on the /<slug>.md suffix and prints the first hit.
• curl -s https://code.claude.com/docs/llms.txt | grep -n cli-reference.md returns the plugins page at line 155 and the top-level page at line 267 (checked 2026-09-29).
• Found by a post-merge review of feat(claude-config): shared upstream doc fetcher with per-run manifest #5440, which merged while both Claude review lanes failed on a 429 rate limit.

Acceptance criteria

• index_link <slug> returns only the exact top-level page ($ORIGIN/docs/en/<slug>.md, or $ORIGIN/docs/<slug>.md), never a nested page that shares the basename.
• A fetch-docs.test.sh case uses an index that lists plugins/<x>.md before <x>.md and asserts that the manifest URL is the top-level page.
• --discover reports the top-level page as in the index for that fixture.
• claude-config version bump and CHANGELOG entry.