Migrate MCP Apps support from insiders mode to feature flag with insiders opt-in

[mattdholloway]

Summary

This pull request transitions MCP Apps from an insiders-only experimental feature to a general feature flag (remote_mcp_ui_apps) that can be enabled independently.

Why

Closes https://github.com/github/copilot-mcp-core/issues/1471

What changed

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)

[Copilot]

Pull request overview

This PR migrates MCP Apps (interactive UI forms) enablement from “insiders mode” to an explicit feature flag (remote_mcp_ui_apps), while keeping backward compatibility by treating insiders mode as implying the flag.

Changes:

• Introduces github.MCPAppsFeatureFlag (remote_mcp_ui_apps) and wires it through stdio/HTTP feature checking with an insiders-mode transitional fallback.
• Adds inventory builder support (WithMCPApps) to strip/keep MCP Apps UI metadata (Meta["ui"]) based on the feature flag.
• Updates tool behavior gates and documentation to reference the feature flag instead of insiders mode.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pkg/inventory/registry_test.go	Renames/extends tests for MCP Apps gating and meta stripping; adds stripMetaKeys coverage.
pkg/inventory/builder.go	Adds WithMCPApps, separates insiders stripping from MCP Apps UI metadata stripping, and introduces stripMetaKeys.
pkg/http/server.go	Whitelists remote_mcp_ui_apps for X-MCP-Features and adds transitional insiders behavior in feature checker.
pkg/http/handler.go	Applies WithInsidersMode and WithMCPApps to the inventory builder based on request context/header features.
pkg/github/pullrequests.go	Gates PR UI form behavior on remote_mcp_ui_apps feature flag instead of insiders mode.
pkg/github/pullrequests_test.go	Updates UI-gate test setup to use feature checker-based deps rather than InsidersMode.
pkg/github/issues.go	Gates issue UI form behavior on remote_mcp_ui_apps feature flag instead of insiders mode.
pkg/github/issues_test.go	Updates UI-gate test setup to use feature checker-based deps rather than InsidersMode.
pkg/github/feature_flags.go	Adds the exported MCPAppsFeatureFlag constant.
internal/ghmcp/server.go	Ensures stdio “insiders mode” also enables remote_mcp_ui_apps; registers UI resources based on the flag.
docs/server-configuration.md	Documents feature flags and MCP Apps configuration via X-MCP-Features / --features.
docs/insiders-features.md	Removes MCP Apps from insiders-only list; points to feature-flag configuration.