[Feat] Add Buildkite built-in integration

[roomote-roomote]

​Opened on behalf of @daniel-lxs. Follow up by mentioning @roomote-roomote, in the web UI, or in Telegram.

What changed

• Added Buildkite to the existing MCP_INTEGRATIONS registry using the provider-enforced https://mcp.buildkite.com/mcp/readonly endpoint, deployment-scoped OAuth/DCR, the exact OAuth resource, and the read scope.
• Reused the existing Settings integration list and generic OAuth connection flow. Integrations.tsx continues to render directly from MCP_INTEGRATIONS; this adds no new list, section, or special UI.
• Wired the shared URL-detection registry into active Slack agent context so Buildkite links in authored text, block links, and attachment title links trigger built-in integration setup guidance while public, API, and MCP URLs are excluded.
• Added the Simple Icons Buildkite mark, self-setup and Slack recommendation metadata, task setup guidance including SSO/IP-allowlist caveats, focused registry/runtime tests, and public docs.

Why this change was made

Buildkite's public OAuth metadata confirms dynamic client registration and PKCE. Its hosted read-only endpoint provides a safer built-in default without inventing a tool-name allowlist.

Impact

Operators can connect Buildkite once from Settings > Integrations, and pasted or unfurled organization links now give Slack tasks an explicit setup recommendation when the built-in integration is unavailable. URL parsing uses bounded punctuation handling rather than the regex flagged by CodeQL, with direct repeated-punctuation coverage. Focused URL-detection, Slack-context, attachment-unfurl, and event-normalization tests passed alongside full check-types, lint:fast, and git diff --check. Live OAuth consent, tool discovery, and credential-backed Buildkite calls were not performed.

Related PRs

• Cloudflare #2992
• Stripe #2995
• CircleCI #2993

[roomote-community]

No new code issues found. See task

• Buildkite Slack URL-detection metadata is now consumed, so pasted Buildkite URLs trigger setup recommendations.
• Trailing-punctuation cleanup now runs in linear time on crafted Slack URL candidates.
• Attachment-only Buildkite title_link URLs now trigger setup recommendations.
• Repeated-punctuation coverage now exercises the stripping path.

_{Reviewed 89ed506}