fix(ci): chain-image parses tag from scrape-pack run-name

[laradji]

Closes #218.

Summary

• chain-image.yml's Resolve tag from upstream run step was reading gh api .../runs/<id> --jq '.inputs.tag', which always returns null — the runs REST endpoint does NOT expose workflow_dispatch inputs. Latent since refactor(ci): split docker image publish out of scrape-pack into its own concern-per-file workflow #209; surfaced after fix(ci): chain-release uses PAT so scrape-pack completion fires chain-image #217 made chain-image fire reliably (every release v0.7.0–v0.7.3 needed a manual gh workflow run docker-publish.yml because chain-image's auto-dispatch was always skipped).
• scrape-pack.yml gains a top-level run-name: that embeds inputs.tag: scrape-pack (v<x.y.z>) for tagged runs, scrape-pack (cache-warm) for empty-tag dispatches.
• chain-image.yml parses the tag back out of github.event.workflow_run.display_title instead of hitting the runs API. Stable-tag regex (^v[0-9]+\.[0-9]+\.[0-9]+$) and cache-warm skip gate preserved exactly. Old misleading comment about "the inputs field on the run JSON" deleted per the issue AC.

Test plan

• python3 -c "import yaml; yaml.safe_load(open('.github/workflows/scrape-pack.yml'))" — OK
• python3 -c "import yaml; yaml.safe_load(open('.github/workflows/chain-image.yml'))" — OK
• Regex sanity-checked locally against four inputs: scrape-pack (v0.7.4) → tag=v0.7.4, scrape-pack (cache-warm) → cache-warm gate, scrape-pack (v1.2.3-rc1) → stable-tag-filter skip, bare scrape-pack (legacy in-flight runs) → cache-warm gate.
• After merge: probe with gh workflow run scrape-pack.yml --ref main -f lib="" -f tag="" → chain-image's Resolve-tag step should log "cache-warm path; skipping".
• After merge: probe with gh workflow run scrape-pack.yml --ref main -f lib="" -f tag=v0.7.3 → chain-image's Dispatch docker-publish.yml step conclusion should be success (not skipped), and a fresh docker-publish run should appear shortly after.
• End-to-end validation: tag v0.7.4 — full chain (release → chain-release → scrape-pack → chain-image → docker-publish) must complete with no operator dispatch. This is the issue's Definition of Done.