ci: fix Context7 refresh — branch-name addressing + default-variant handling

The post-merge auto-fired runs from #1948/#1949/#1950 all returned 4xx
errors. Diagnostic curls against the live Context7 API revealed two
issues:

1. The 'branch' parameter addresses variants by their underlying Git
   branch name (rolling/circinus/sagitta), not by their tag display name
   (rolling/1.5/1.4). Sending 'branch: "1.5"' returns:
     HTTP 404 {"error":"branch_not_found","message":"Branch '1.5' not found"}

2. The default variant refreshes when the 'branch' field is omitted
   entirely. Sending 'branch: "rolling"' returns:
     HTTP 400 {"error":"branch-not-found","message":"Failed to refresh library"}
   But omitting the field returns:
     HTTP 200 {"message":"Refresh started successfully"}

3. (Bonus, validating #1951 was wrong direction.) The libraryName must
   include the leading slash: 'libraryName: "/vyos/vyos-documentation"'
   per Context7's docs. Without the slash returns:
     HTTP 404 {"error":"library_not_found"}
   This commit restores the leading slash that #1951 incorrectly removed.

Changes:
- Restore leading slash on libraryName ('/' + github.repository).
- Drop the tag-name mapping in the case statement; pass head_branch
  directly as the branch value.
- Omit the 'branch' field when head_branch is 'rolling' (default variant).
- workflow_dispatch input renamed from 'version' to 'branch'; choice
  options changed from [rolling, '1.5', '1.4'] to [rolling, circinus, sagitta].
- Simplified concurrency expression (no longer needs the rewrite chain).
- Documentation comment updated to explain the branch-name addressing.

Spec: ~/.claude/specs/2026-05-10-context7-github-actions-integration-design.md
This is the 'fallback mapping' path that the spec's variant table already
documented as a contingency — pre-flight confirmed it's the correct path.

🤖 Generated by [robots](https://vyos.io)

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