Description

Allow parsec start to attach to an existing branch instead of always creating a new one from the base branch.

Problem

Currently, parsec start always creates a new branch from main/master (or --base). When a developer has already been working on feature/CL-2208 outside of parsec, they can't bring it under parsec management without parsec adopt, which requires the branch to already exist as a worktree.

The gap: if you want to continue working on an existing remote branch inside a new parsec-managed worktree, there's no clean path.

Proposed Solution

# Create a parsec worktree using an existing branch
parsec start CL-2208 --branch feature/CL-2208

# If branch exists on remote but not locally, fetch and track it
parsec start CL-2208 --branch origin/feature/CL-2208

Behavior

• --branch <name> tells parsec to use this existing branch instead of creating {prefix}/{ticket}
• If the branch exists locally, create a worktree pointing to it
• If the branch exists only on remote, fetch and create a local tracking branch
• If the branch doesn't exist anywhere, error out (use regular parsec start for new branches)
• The workspace is fully managed by parsec after this (ship, sync, status, etc.)

Difference from parsec adopt

• adopt imports the current directory's branch into parsec
• --branch creates a new worktree from a specified existing branch

Use Cases

1. Taking over a colleague's branch for review/continuation
2. Resuming work on a branch that was started before using parsec
3. Working on a branch that was created by CI or another tool
4. Feature branches that span multiple tickets

Priority

Medium — frequent pain point when mixing parsec with regular git workflow