chore(ci): retire one-shot tombstone publish jobs

[Dongbumlee]

Summary

The agentops-toolkit → agentops-accelerator deprecation tombstones were one-shot publishes for v0.3.0 / v0.3.1 (PyPI manually uploaded, VSIX shipped via Marketplace). Per the user directive "once we put the tombstone (v0.3.0), we don't need to care of it anymore after new releasing" and "make sure our release pipeline will not touch any tombstone works anymore", this PR removes every tombstone job, prose mention, and orphaned source file from the repo.

Refs: #181

Workflow deletions

release.yml (548 → 281 LOC)

• ❌ build-pypi-tombstone
• ❌ publish-tombstone-testpypi
• ❌ verify-tombstone-testpypi
• ❌ publish-tombstone-pypi
• ❌ publish-tombstone-vsix

staging.yml (474 → 208 LOC)

• ❌ build-pypi-tombstone
• ❌ publish-tombstone-testpypi
• ❌ verify-tombstone-testpypi
• ❌ publish-tombstone-vsix-prerelease

cut-release.yml (211 → 173 LOC)

• ❌ Sub-blocks updating tombstones/vscode/package.json, tombstones/pypi/pyproject.toml, tombstones/vscode/CHANGELOG.md
• ❌ Tombstone paths removed from git add in commit step
• ❌ Tombstone bullets removed from PR body markdown

github-release simplification

- needs: [publish-pypi, publish-vsix, publish-tombstone-pypi, publish-tombstone-vsix]
- if: always() && needs.publish-pypi.result == 'success' && needs.publish-vsix.result == 'success'
+ needs: [publish-pypi, publish-vsix]
+ if: needs.publish-pypi.result == 'success' && needs.publish-vsix.result == 'success'

The always() guard was dead code: with both deps now required, GitHub Actions implicitly skips the job whenever either need is non-success, and the explicit == 'success' checks would have caught it anyway. Verified behavior-equivalent across all four outcome combinations (both-success, partial, cancelled, upstream-failure).

Repo hygiene (per Critic findings)

• 🗑️ scripts/verify_tombstones.py — already-done one-shot harness; expected 6 release assets but the new pipeline ships 3, so it would fail with misleading errors.
• 🗑️ docs/verifying-tombstones.md — referenced deleted CI jobs by name + line numbers.
• 🗑️ tombstones/pypi/ (entire dir)
• 🗑️ tombstones/vscode/{LICENSE,CHANGELOG.md,icon.png,package.json,README.md,tsconfig.json,.vscodeignore,src/} — orphaned source no workflow consumes.
• ✅ tombstones/vscode/CDN_DEPRECATION_REQUEST.md kept — template for the still-pending Microsoft CDN deprecation request.

Line-ending consistency

• ➕ .gitattributes pinning *.yml, *.yaml, *.sh, *.md, *.py to LF so Windows clones with core.autocrlf=true don't repeatedly flip workflow files.
• 🔄 Normalized _build.yml and ci.yml (previously CRLF) to LF — all 7 files in .github/workflows/ now share one convention.

Verification

• ✅ YAML parses for all 5 workflow files
• ✅ Zero tombstone refs in .github/, scripts/, docs/, or top-level docs (only in CHANGELOG historical entries and CDN_DEPRECATION_REQUEST.md)
• ✅ All artifact wiring (dist + vsix) preserved; no dangling download-artifact references
• ✅ Next-release walk-through (v0.3.1 tag → build → publish-testpypi → verify-testpypi → publish-pypi → publish-vsix → github-release) DAG is intact

Next steps (out of scope for this PR)

• File Microsoft CDN deprecation request per tombstones/vscode/CDN_DEPRECATION_REQUEST.md to retire the legacy Marketplace listing

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