fix(vscode): guard subagent promotion edge cases

[marius-kilocode]

What Problem This Solves

Foreground subagent promotion could show a stale action after a child session was resumed, and the child-specific promotion endpoint did not enforce the background-subagent capability flag.

Why This Change Was Made

• Show the promotion action only on the latest task part for each child session.
• Enforce experimentalBackgroundSubagents in the promotion handler.
• Propagate the server capability to the VS Code webview and fail closed when capability discovery is unavailable.

User Impact

Users no longer see a dead promotion action on stale task cards. Disabling background subagents prevents promotion in both the backend and the webview.

Evidence

• Extension typecheck, lint, formatting, compile, Knip, and focused regression tests passed.
• CLI typecheck and background route/task tests passed.
• OpenCode annotation and Kilo marker checks passed.
• Regression coverage includes resumed-child ordering, disabled capability visibility, and disabled promotion routing.