feat(taskctl): make start command resilient to existing branches

[randomm]

Problem

When taskctl start <issue> is run after a previous failed attempt, it fails with:

Failed to create feature branch feature/issue-N: fatal: a branch named 'feature/issue-N' already exists

This blocks retrying failed pipelines without manual cleanup.

Expected Behavior

taskctl start should:

• Option A: Detect and clean up stale branches/worktrees automatically
• Option B: Resume work on existing branches
• Option C: Use unique branch names per attempt (e.g., feature/issue-N-attempt-2)

Current Workaround

Manual cleanup:

git branch -D feature/issue-N
git worktree remove .worktrees/task-name
taskctl start N

Quality Gates (Non-Negotiable)

• TDD: Write tests for branch creation logic
• Linting passes
• Local verification: all tests pass

Acceptance Criteria

• taskctl start succeeds even if branch already exists
• No manual cleanup required
• Previous failed attempts do not block new attempts

[randomm]

Additional Context

This issue is related to #382. When taskctl start fails due to existing branch, users cannot retry pipelines without manual cleanup.

Current workaround:

git branch -D feature/issue-N
taskctl start N

Better solution: Make taskctl start resilient by:

1. Detecting existing branch → offer to resume or recreate
2. Or using unique branch names per attempt

This blocks users from testing fixes like #382.

[randomm]

Closing: fixed in #404