Drop the `parseWorkspaceRootMcpConfiguration` change that accepted the
Copilot `"local"` type, along with its tests. #340034 makes the same
change, so this PR now only covers Copilot CLI user configuration
discovery and keeping Copilot-home servers from being forwarded.

Refs #339955, #340161

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…ng it (#340181)

<!-- Thank you for submitting a Pull Request. Please:
* Read our Pull Request guidelines:

https://github.com/microsoft/vscode/wiki/How-to-Contribute#pull-requests
* Associate an issue with the Pull Request.
* Ensure that the code is up-to-date with the `main` branch.
* Include a description of the proposed changes and how to test them.
-->

Part of #340161. Together with #340034, fixes #339955.

Servers migrated from user `mcp.json` to `~/.copilot/mcp-config.json`
stopped showing up in local chat. This PR makes the local harness
discover that file by default, without forwarding its servers to other
agents or hosts. The workspace `.mcp.json` side (accepting `"type":
"local"`) is handled in #340034.

## Changes

**`~/.copilot/mcp-config.json` is discovered by default**

Migration moves user-level servers into the Copilot CLI user config, but
the `copilot` source of `chat.mcp.discovery.enabled` was off by default.

- Added `defaultDiscoverySourceEnablement`, with only `copilot: true`.
Claude Desktop, Cursor and Windsurf stay off.
- Added `isDiscoverySourceEnabled`, used by `_isDiscoveryEnabled`. A
source the user hasn't set falls back to its default. An explicit
`false` and the legacy boolean form are still respected.
- The configuration service deep-merges a partial user value (e.g. `{
"claude-desktop": true }`) with the default object, so `copilot` stays
on in that case too.
- Updated the setting description.

**Copilot-home servers are never forwarded to other agents or hosts**

- The same-machine Copilot CLI session still reads the file itself
(`RuntimeDiscovered`).
- In every other agent-host session (Claude, Codex, or Copilot on
another remote authority) they are `NotDelivered`, with a new
`CopilotHomeNotForwarded` reason and message.
- This keeps literal `env` and header credentials in `mcp-config.json`
from being copied to another provider or machine without approval, now
that the source is on by default.

## Notes for reviewers

- **Duplicates:** with the default `chat.mcp.collisionBehavior:
disable`, a server with the same name in user `mcp.json` (order 200)
takes priority, and the `mcp-config.json` copy (order 400) is disabled.
- **Servers appearing for existing CLI users:** people with servers in
`~/.copilot/mcp-config.json` will now see them in local chat. This
includes the remote machine's file in remote windows. These servers are
trusted only after the user approves them, so expect a trust prompt.
- **Lost forwarding:** users who had opted in to the `copilot` source
lose forwarding of these servers to Claude/Codex sessions and other
remote hosts.
- **Explicit opt-outs:** users who set `"copilot": false`, or the legacy
`false`, will still not see their migrated user servers.

## How to test

1. With `chat.mcp.discovery.enabled` unset, add a server to
`~/.copilot/mcp-config.json`. It should appear as "GitHub Copilot CLI".
2. Set `chat.mcp.discovery.enabled` to `{ "copilot": false }` or
`false`. The server should disappear.
3. Open a Claude or Codex agent-host session. The Copilot-home server
should show as unsupported with the "not shared with other agents or
remote hosts" explanation.

## Validation

- `npm run typecheck-client`: passed
- `npm run build-fast -- --client-only`: passed
- `./scripts/test.sh --reporter dot --run
src/vs/platform/mcp/test/common/mcpWorkspaceConfiguration.test.ts --run
src/vs/platform/mcp/test/common/mcpResourceScannerService.test.ts --run
src/vs/workbench/contrib/mcp/test/common/workspaceDotMcpDiscovery.test.ts
--run
src/vs/workbench/contrib/mcp/test/common/nativeMcpDiscoveryAdapters.test.ts
--run
src/vs/workbench/contrib/chat/test/browser/agentSessions/agentHostMcpServerSupport.test.ts
--run
src/vs/workbench/contrib/chat/test/browser/agentSessions/resolveCustomizationRefs.test.ts
--run
src/vs/workbench/contrib/chat/test/browser/aiCustomization/mcpServerCustomizationMigration.test.ts`:
181 passing
- `npm run eslint -- <changed files>`: clean