fix(worktree): claim non-prompt tasks before worktree creation to prevent duplicate-worktree race

[IBondarenko-iwg]

Linked issue

Closes #536

Summary of changes

Non-prompt auto-dispatch tasks (script, mcp, task_gen) claimed in-progress status after worktree creation. Two concurrent runners could both observe the same task as todo via the lock-free Get-NextWorkflowTask, both call git worktree add to the same path, and the second would fail with fatal: '...' already exists.

Root fix: Added a pre-worktree claim guard for non-prompt tasks mirroring the existing prompt-task guard. Calls Invoke-TaskMarkInProgress (HTTP POST with server-side mutex) before any worktree I/O. no_op=true in the response signals the task was already claimed — the guard fetches a replacement task and breaks to the outer loop so full task_gen/prompt-template recovery runs on it. Removed the post-worktree Set-TaskInProgressForExecutorDispatch call, now superseded.

Additional hardening (from code review):

• Re-syncs $processData.task_id / $env:DOTBOT_CURRENT_TASK_ID after the guard in case $task was replaced during retry
• Fatal errors (unexpected status, non-HTTP exceptions) re-throw instead of silently retrying
• 2s back-off before outer-loop continue when all claim attempts exhaust
• On task swap, always breaks to outer loop so the replacement goes through task_gen/prompt_template recovery
• Updated Test-ProcessDispatch.ps1 assertion to match Invoke-TaskMarkInProgress instead of the removed function name

Testing notes

1. Unit — pwsh tests/Test-ProcessDispatch.ps1: the "legal executor status transitions" assertion now validates the claim guard is present.
2. Race reproduction — start two task-runner processes against a workflow with at least one pending non-prompt task. Before this fix both runners race to git worktree add; after, the second detects no_op=true and picks up a different task.
3. Standalone runner — verify a single runner (-Slot not set) still claims and executes non-prompt tasks normally; the guard is unconditional and must not regress the single-runner path.

Checklist

• Tests added or updated
• Linked issue exists
• Follows the contribution guide

[IBondarenko-iwg]

@carlospedreira Heads up: potential merge conflict with #538 (feature/issue-510). Both PRs modify Invoke-WorkflowProcess.ps1 at the same hunk (~line 1485). This PR removes Set-TaskInProgressForExecutorDispatch and inserts the claim guard block there; #538 also adds lines in that area. No semantic overlap — whoever merges second will need a manual hunk combine.