Summary

The daily "Update Star History & Contributors" workflow commits regenerated chart
SVGs directly to main. Since branch protection now requires all changes to go
through a PR, that push is rejected and the workflow fails every day
(see run 30604986667). Beyond the failure, machine-generated daily commits also
pollute main's history (~365 commits/year of chart refreshes).

This PR moves the chart assets off main entirely:

• The workflow now renders SVGs into $RUNNER_TEMP and force-pushes them to an
  orphan branch chart-assets as a single snapshot commit. The branch carries
  only the latest three SVGs — no history accumulates, and the branch is created
  automatically on first run.
• Both READMEs reference the charts via
  https://raw.githubusercontent.com/Stack-Cairn/LiveAgent/chart-assets/<name>.svg,
  so main never needs another chart commit.
• The now-unreferenced SVGs under docs/images/ are removed.

If a future run fails, the previous SVGs remain on the branch and the READMEs
keep showing yesterday's charts — no broken images.

Change scope

• Modules: GitHub workflows + README docs (no product code)
• Key paths:
  • .github/workflows/update-star-history.yml
  • README.md / README.zh-CN.md (chart image URLs)
  • docs/images/star-history-*.svg, docs/images/contributors.svg (deleted)

Screenshots / preview

N/A — the rendered README section is visually unchanged (same charts, same
light/dark handling); only the image source URLs move to the chart-assets
branch. Chart appearance itself is verifiable on the repo front page after the
first post-merge workflow run.

Verification

• rg confirms no remaining references to the deleted docs/images SVGs
  anywhere in the repo.
• Workflow logic reviewed: SVGs are generated into $RUNNER_TEMP (clean
  worktree before git switch --orphan), and the force-push is explicitly
  scoped to origin chart-assets; permissions unchanged (contents: write).
• Post-merge steps: manually dispatch the workflow once to create the
  chart-assets branch (README images 404 until then), and confirm the branch
  protection ruleset matches main only so it does not block pushes to
  chart-assets.

Pre-submit checklist

• A requirement issue is linked (or this is a trivial fix that needs no issue, as explained in the summary).
• Synced with the target branch; no merge conflicts.
• The change is focused, with no unrelated modifications.
• No secrets, tokens, or personal data included.
• Docs are updated for changes affecting user behavior, deployment, or configuration.