[FEATURE]: Explicitly add MCP server config to project level opencode.json

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Re-log of #30933, which was auto-closed after 60 days of inactivity. The request is unchanged and still unimplemented on dev.

Ask: opencode mcp add should support an explicit CLI parameter to choose between project-level (opencode.json in the project directory) and global (~/.config/opencode/opencode.json) config, and should always prompt for the target scope when no parameter is given.

Current behaviour (verified on dev, packages/opencode/src/cli/cmd/mcp.ts):

• The "Current project" vs "Global" prompt is gated on if (project.vcs === "git") (line 516). Outside a git repository, the interactive flow never asks and the server silently lands in the global config — even though the project config file would otherwise be created for you.
• There is no explicit flag to force either scope, so automation/scripts can't target a project deterministically.

Notes:

• PR #44803 (feat(mcp): scoped server config, project approval, and keychain storage) adds opencode mcp add --scope local|project|user, which would cover this — still open/unmerged at time of filing; linking for reference.
• Expected: e.g. opencode mcp add --project / --global (or a single --scope flag) forces the target config; without the flag, always prompt for scope regardless of git state.