ci: track Context7 LTS variants by branch instead of moved git tags (#2127)

* ci: track Context7 LTS variants by branch instead of moved git tags

Switch the Context7 1.5/1.4 variants from tag-backed to branch-backed:

- context7.json: previousVersions entries `{tag:"1.5"}`/`{tag:"1.4"}` →
  `{branch:"circinus"}`/`{branch:"sagitta"}`. Context7 now indexes the
  LTS branch HEADs directly.
- Delete update-version-tags.yml. Its sole purpose was force-moving the
  git tags 1.5/1.4 to circinus/sagitta HEAD so the tag-backed Context7
  variants tracked the branches. With branch-backed variants Context7
  tracks the HEAD natively, so the tag indirection — and its re-run
  "moves the tag backward" footgun — is gone.
- context7-refresh.yml: trigger directly on push to rolling/circinus/
  sagitta (docs/** + context7.json paths) instead of chaining off the
  deleted tag-mover via workflow_run; refresh payload uses
  `branch:"circinus"`/`branch:"sagitta"` for the non-default variants.

Operator follow-up (post-merge): the circinus/sagitta branch variants
must be registered in the Context7 dashboard (a refresh by branch 404s
until Context7 crawls the new config), and the now-unused git tags
1.5/1.4 can be deleted.

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

* ci: harden jq repo interpolation in context7-refresh

Adversarial review (agy): use the $GITHUB_REPOSITORY runner env var
(mapped as REPO) instead of inlining ${{ github.repository }} into the
jq command string, matching the safe env-mapping pattern already used
for EVENT_NAME/REF_NAME. github.repository is validated and low-risk,
but this structurally removes the interpolation-into-shell surface.

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

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