Release v0.21.6-preview.0

What's Changed

• feat(browser-ext): add alpha readiness diagnostics by @yiliang114 in #6739
• docs: document headless Goal workflows by @DragonnZhang in #8503
• feat(core): guide recording of PR, issue, and comment URLs as artifacts by @wenshao in #8453
• ci: route trusted-author fork PRs and no-checkout jobs to the ECS pool by @wenshao in #8502
• fix(web-shell): keep pending background agents active by @carffuca in #8413
• fix(desktop): rotate Tauri updater signing key by @yiliang114 in #8511
• fix(web-shell): add explicit ::selection for message content in Firefox by @C0d3N1nja97342 in #8417
• refactor(core): move review skill incident narratives to DESIGN.md by @wenshao in #8499
• fix(core): harden Qwen 3.8 reasoning effort wire shape by @wenshao in #8488
• perf(review): issue independent setup calls in one response by @wenshao in #8487
• fix(review): stop the reverse-audit loop while there is still time to report by @wenshao in #8468
• fix(autofix): normalize paginated fetches to one flat array per file by @wenshao in #8438
• feat(web-shell): bind plan approval to its Todo revision by @yiliang114 in #8393
• fix(core): resolve DashScope thinking-knob conflicts by family (#8488 round 2) by @wenshao in #8536
• fix(cli): stop review test-efficacy tests depending on ambient tmpdir vitest by @wenshao in #8537
• feat(serve): add a required external tool guard provider by @chiga0 in #8125
• fix(desktop): codesign ripgrep and node binaries before tauri build by @yiliang114 in #8518
• fix(core): reuse prompt cache for multimodal compression by @DragonnZhang in #8419
• fix(cli): preserve Qwen Review startup version in footers by @yiliang114 in #8431
• fix(serve): detect lineEnding across the file, not the returned slice by @doudouOUC in #8383
• perf(core): clear tool results to a low watermark to preserve prompt cache by @doudouOUC in #8464

Full Changelog: v0.21.5...v0.21.6-preview.0