[openai-developers] Add a DevDay guide and agenda extension

[cching-openai]

DevDay attendees need to check the public program, plan their day, and discuss their choices in ChatGPT. Add one narrowly triggered devday-guide skill and a focused agenda extension to OpenAI Developers.

The skill verifies the official public event site and leaves unpublished sessions, rooms, and end times unknown. The existing MCP server gains one open_devday_agenda tool and one registered MCP App resource. A thread entrypoint and display preference request the conversation side panel on supporting hosts. The view uses the launch result directly, without refetching or bundling a schedule.

Attendees select activities, change timezones, and click Use plan in chat to attach a snapshot to their next message. The view handles attachment removal without clearing newer choices or reattaching automatically. Changing the selection asks the attendee to attach an updated snapshot. Nothing sends messages, reserves sessions, or changes a calendar. Downloads use the host's supported download action. The same HTML still works through the offline renderer where the MCP App is unavailable.

The skill and extension source total about 43 KB. There are no new packages, servers, connectors, installation builds, analytics, external fonts, browser network calls, or global navigation entries. The small JSON-RPC bridge implements MCP Apps initialization, tool-result delivery, display mode, model context, host downloads, and teardown. Existing branding and default prompts stay the same. Bump the plugin from 1.2.3 to 1.2.4.

Validation:

• Five extension tests passed, including the actual stdio server, registered resource, metadata, input rejection, isolated calls, parent/origin validation, launch-result rendering, context attachment/removal, remount restoration, and capability fallbacks.
• The served HTML passed offline DOM checks for safe text rendering, selection, filtering, timezone rollover, attachment, removal, and host-mediated download.
• Seven Python renderer tests passed. Existing API-key tests: 28 passed and two monorepo-only fixtures skipped.
• Current plugin and skill validators passed. git diff --check passed.
• A fresh skill walkthrough correctly distinguished submitted plan context from later unsubmitted panel edits and declined unsupported reservations.

Remaining verification:

• This is source and local protocol validation, not a verified ChatGPT installation. The current task lacks the Plugin Creator publishing/install tools needed to provision a private full-plugin test release.
• Real ChatGPT loading, side-panel placement, and the conversation round trip must be checked with that release. A complete manual test flow is in the README.
• Previous offline layout proofs are not browser screenshots. The browser policy blocked local HTML navigation; no workaround was used.

After DevDay, the same skill prioritizes verified recordings and examples and hands implementation work to the existing developer skills. Past agendas are labelled as archives.