feat(mcp): replace shiny.mcp.* options with mcpConfigure()

[schloerke]

Summary

Replaces the seven options(shiny.mcp.*) MCP App options with a single validated mcpConfigure() function backed by .globals$mcp, tightening how an app opts into and configures MCP support. Also renames the session helper mcpToolInput() → mcpUpdates(), and routes the model's app-opening arguments through Shiny's bookmark RestoreContext so inputs initialize flash-free, while post-init argument updates flow through the reactive mcpUpdates() channel.

Motivation: options() is an unregulated, process-global namespace (no validation, no discoverability, silent typos, cross-app leakage). mcpConfigure() is one gated, documented, autocomplete-friendly entry point.

Public API

mcpConfigure(
  appId        = NULL,   # unique id for gateway merging; derives tool name + resource URI
  description  = NULL,   # what the model reads to decide whether to open the app
  arguments    = NULL,   # named list of ellmer type_*() objects — the model-settable allow-list
  direct       = TRUE,
  displayModes = c("inline", "fullscreen", "pip"),
  origin       = NULL,
  stdio        = FALSE,
  enabled      = TRUE    # LAST: an app is served over MCP only if it calls mcpConfigure()
)

• Opt-in safeguard: no mcpConfigure() call ⇒ MCP is never served. Invalid appId now errors (was warn-and-ignore).
• Tool name derived from appId (open_shiny_app / open_<appId>_app) — drops the manual tool$name knob; gateway uniqueness is automatic.
• arguments uses ellmer types (consistent with registerMcpTool()), published as the tool inputSchema and enforced as an allow-list on both the init and update paths.

Argument delivery

• Init → the model's opening arguments are filtered to the declared allow-list, injected into the init frame, and applied via Shiny's RestoreContext (standard inputs auto-restore via restoreInput(), no flash; onRestore() covers non-input state). Works without the app enabling bookmarking (the restore-callback observers were extracted into createRestoreObservers(); bookmarking behavior is unchanged).
• After-init → live re-steering flows through the reactive mcpUpdates(), wired by the author with updateXxxInput().

Notes

• Experimental/unreleased feature: clean removal of options(shiny.mcp.*), no deprecation shims. The three sendCustomMessage channel names (shiny.mcp.updateModelContext/sendMessage/requestDisplayMode) are protocol channels, not options, and are unchanged.
• Design spec, plan, and spike notes are in mcp/ (design-mcpConfigure.md, plan-mcpConfigure.md, notes-restore-spike.md).

Testing

• MCP suite: 246 pass / 0 warn / 1 CRAN-skip. Bookmark suite: 54 pass (regression check — extraction is behavior-preserving). JS bridge: 27 pass (incl. init-restore injection seam tests). Full devtools::test(): 3334 pass (14 pre-existing, unrelated test-zzz-st2-download.R failures).
• NEWS.md bullet references PR #XXXX — replace with this PR number before merge.

🤖 Built with subagent-driven development (9 planned tasks + 1 final-review fix, each independently reviewed).