feat: add coderm.workbench.openReadme command

[j4rviscmd]

Summary

Adds a new Coderm command coderm.workbench.openReadme ("Open README") that opens the workspace root's README.md (first workspace folder) on a best-effort basis. If README.md is absent or no workspace folder is open, it falls back to the upstream workbench.action.files.newUntitledFile command.

Behavior

• README.md present in the first workspace folder → opens it (pinned tab)
• README.md absent, or empty workspace (no folders) → runs workbench.action.files.newUntitledFile (new untitled file)
• Only the first workspace folder is consulted — multi-root workspaces do not scan every folder (intentional simplicity trade-off)

Files

• src/vs/workbench/contrib/coderm/browser/openReadme.ts (new) — Action2 + registerAction2
• src/vs/workbench/contrib/coderm/browser/coderm.contribution.ts — side-effect import
• README.md / README.ja.md — command table entry

The fallback delegates to the upstream newUntitledFile command via ICommandService.executeCommand(NEW_UNTITLED_FILE_COMMAND_ID) rather than re-implementing it, so upstream behavior changes are tracked for free.

Follows Coderm conventions: command lives under contrib/coderm/browser/, no default keybinding (users assign their own, like other Coderm commands).

Related to #105 — this adds a manual command variant. #105 additionally asks for automatic open on startup when no editor pane exists (not covered here).

Checklist

• Build passes (gulp compile, 0 errors)
• ESLint passes (0 errors)
• README.md / README.ja.md synced