bug(index): worktree_strategy="delegate" silently ignored; no validation when strategy="auto" but branch unset

[apex-skyner]

Problems

1. worktree_strategy: "delegate" not supported

The dev-agent-routing skill uses:

worktree_strategy: 'delegate'  // plugin creates worktree automatically

But the WorktreeStrategyEnum only has "auto" and "off". Any delegate value causes a schema validation error from OpenClaw before the tool even runs. delegate should be an alias for auto (the plugin IS doing the worktree creation).

2. No validation: auto strategy without branch

When worktree_strategy === "auto" but branch is not provided, the code silently falls through and works in the original workdir without creating a worktree. The user intended isolation and gets none — a silent footgun.

// current — silently no-ops on worktree creation when branch is missing
if (worktreeStrategy === 'auto' && params.branch) {
  ...
}

Should return an error if auto is requested without a branch.

3. Sessions never pruned from memory

The in-memory Map<string, ManagedSession> in SessionManager accumulates indefinitely. Long-running OpenClaw gateway processes will grow without bound. Completed sessions older than ~24h should be pruned automatically.

Fix

• Add "delegate" as an alias for "auto" in WorktreeStrategyEnum
• Return { error: ... } when strategy === "auto" || "delegate"" but branch` is absent
• Add a background pruning interval to SessionManager that removes sessions ended >24h ago