ci: address Context7 LTS variants via tag field, not branch field

Post-#1961, the rolling auto-refresh (default variant, no field) succeeds,
but workflow_dispatch for branch=circinus/sagitta still 404s. Further
empirical curls against the live API revealed:

  POST /api/v1/refresh
  {"libraryName":"/vyos/vyos-documentation","branch":"circinus"}
  → HTTP 404 {"error":"branch_not_found","message":"Branch 'circinus' not found"}

  POST /api/v1/refresh
  {"libraryName":"/vyos/vyos-documentation","tag":"1.5"}
  → HTTP 200 {"message":"Refresh started successfully"}

So Context7's API addresses variants by their REGISTRATION TYPE on the
dashboard:
  - default variant (rolling)     → omit both 'branch' and 'tag'
  - tag-backed variants (1.5/1.4) → 'tag' field
  - branch-backed non-default     → 'branch' field

The dashboard shows 'rolling' with a branch icon and '1.5'/'1.4' with tag
icons. The 'branch' field only addresses entries registered as branches;
'tag' addresses entries registered as tags. This is undocumented in the
public GitHub Actions integration page but works against the live API.

Changes:
- Restore the variant mapping (circinus → 1.5, sagitta → 1.4) — that
  matches the actual dashboard variant names. #1961 had dropped this in
  favor of branch-name passthrough, which only worked for the default.
- Switch the non-default payload from 'branch: <name>' to 'tag: <name>'.
- workflow_dispatch input renamed back from 'branch' to 'variant'; choices
  back to [rolling, '1.5', '1.4'].
- Restore the variant-keyed concurrency expression (with rewrite chain).
- Update the documentation comment to record the empirical API semantics.

Spec: ~/.claude/specs/2026-05-10-context7-github-actions-integration-design.md

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