Apply worktree layouts on worktree.opened too (fixes intermittent no-op)

[cloudmanic]

The bug

Worktree auto-layout worked intermittently — sometimes a new worktree got its tabs, sometimes nothing happened.

Root cause: herdr has two worktree events, and we only subscribed to one.

• worktree.created — fired when herdr creates a brand-new worktree.
• worktree.opened — fired when herdr opens an existing worktree into a workspace (e.g. the right-click dialog for a branch that already has a worktree).

We only listened for worktree.created, so opening an existing worktree silently skipped the layout — no handler invocation at all in the plugin log.

The fix

• Subscribe to both worktree.created and worktree.opened, sharing one handler (on-worktree-created → on-worktree, runOnWorktreeCreated → runOnWorktreeEvent).
• Idempotency guard: skip a workspace that already holds more than one pane, so a layout never applies twice — even if both events fire for one workspace or herdr reopens an already-populated one. (A freshly created/opened worktree workspace has exactly one root pane.)
• Fix a stale herdr-plugin.toml comment that still referenced the removed enabled toggle.
• Docs: note both events, and that worktrees must be created/opened through herdr (plain git worktree add / lazygit don't fire herdr events).

Verified live against herdr 0.7.0

Using herdr plugin link of this branch:

Action	Events fired	Result
worktree create (new)	worktree.created ×1	layout applied once
worktree open (existing)	worktree.opened ×1	layout applied (previously skipped)
create	—	confirmed it does not also fire opened (no double-apply)

go build, go test, and go vet all pass.