feat: add isolated manual Bug Server runs for reviewed PRs

[xile611]

Maintainers can manually run Bug Server for a reviewed PR, including fork PRs, by providing its number and full head SHA. They first wait for Bug Server PR Bundle to succeed, then run Bug Server CI from the default branch.

The PR-only workflow builds the exact head with read-only repository permissions and no Bug Server secret. The manual workflow never builds or executes PR code: it validates the current head and the artifact's workflow, event, repository, branch and SHA, downloads that immutable artifact, and submits its index.js bytes using the trusted default-branch client. ZIP paths and links cannot overwrite the client. Artifacts expire after 7 days; rerun the PR bundle workflow when needed.

This removes untrusted execution from the default-branch cache context. The superseded manual build job, cache-mode setting and runtime guard have been removed. Workflow permissions default to contents: read; the manual jobs request only the additional read permissions needed for PR/artifact lookup.

Validation

• On latest commit dac694ecd, CodeQL passed, with no new alerts. Both cache-poisoning alert #45 and missing-permissions alert #46 are marked fixed, without dismissals or disabled rules.
• 36 Node validation tests and 6 Python archive tests passed. Cases include incorrect provenance, expired artifacts, traversal, duplicate files, links and oversized payloads. Both workflows pass actionlint 1.7.12 without ignored diagnostics.
• The real PR bundle build passed. The production resolver and download script located artifact 10483685493; safe extraction produced a 3,210,456-byte bundle. The trusted client uploaded those exact bytes to a local mock API; success, photo-failure and missing-token cases passed. Downloaded JavaScript was never executed locally.
• Required pre-push package tests and unit CI on the implementation commit passed. The final documentation-only commit has also been pushed; its other CI checks may still be running. The existing automatic Bug Server photo test is a separate check and is not a substitute for validating the new manual path.
• Full manual dispatch from the default branch remains a post-merge check; this PR does not claim a live manual Bug Server run has passed.

The README, design notes, execution record and existing Lark maintenance document describe the new workflow and artifact recovery steps.