MCP Apps: foundation (package, capability detection, hello-world status card, fallback policy)

[jernejk]

Umbrella for interactive UI over the MCP server. Depends on the SDK 2.x upgrade (#39, draft PR #76): the ModelContextProtocol.Extensions.Apps package starts at 2.0.

Scope

• Reference ModelContextProtocol.Extensions.Apps, call the builder registration, and serve one embedded HTML resource at a ui://timepro/status address: current user, tenant name, environment (prod/non-prod), accounting feature state, repo mapping for the working directory. Backed by the existing get_location_and_mapping and get_current_user tools; no new data paths.
• Capability detection: attach UI metadata only when the client advertised the io.modelcontextprotocol/ui extension (initialize capabilities, or per-request _meta under 2026-07-28). Terminal clients (Claude Code, Codex CLI) keep today's JSON byte-for-byte; the discovery goldens for a non-UI client must not change.
• Sandbox rules: the page only ever sees tool results; no API keys, no direct HTTP; every write goes back through an existing tool so the CLI-first guards apply.
• Regression harness: a second tools/list golden captured with the UI capability advertised, and a test that the two differ only by UI metadata.
• First-hour check before any real view: confirm Claude Desktop renders a ui:// resource served by a locally launched stdio server (unverified as of 2026-09-13). If it does not, this whole area waits.

Verify against

Claude Desktop first, then VS Code Copilot. Record client versions in docs/mcp-sdk-2-testing.md.

[jernejk]

Child issues, one per CRUD surface:

• MCP Apps: timesheets read (weekly grid over check_week and get_timesheets) #79 timesheets read
• MCP Apps: timesheets create (entry form with project, iteration, category pickers) #80 timesheets create
• MCP Apps: timesheets update, accept and delete from the weekly grid #81 timesheets update, accept, delete
• MCP Apps: leave read (calendar of leave entries and balance status) #82 leave read
• MCP Apps: leave create, update and cancel (form with dry-run preview) #83 leave create, update, cancel
• MCP Apps: accounting read views (reconciliation, unpaid invoices, prepaid, recurring) #84 accounting read views (accounting pack)
• MCP Apps: leave balances import review (unmatched employees and warnings) #85 leave balances import review (accounting pack)

Order: this issue first (hello-world on Claude Desktop decides go/no-go), then #79, then #81, then the rest.