Summary

• Add --branch flag to parsec start that creates a worktree from an existing local or remote branch instead of creating a new one
• If the branch exists only on remote, it is automatically fetched and tracked locally
• If the branch doesn't exist anywhere, a clear error is shown

Closes #46

Changes

• src/git/mod.rs — New worktree_add_existing() function handling local/remote branch resolution
• src/worktree/manager.rs — create() accepts existing_branch parameter
• src/cli/mod.rs — --branch flag on Start command variant
• src/cli/commands.rs — Pass existing_branch through to manager
• README.md — Updated docs with --branch examples
• docs/index.html — Added feature card and comparison table row

Usage

# Attach to a local branch
parsec start CL-2208 --branch feature/CL-2208

# Attach to a remote-only branch (auto-fetches)
parsec start CL-2208 --branch origin/feature/CL-2208

Test plan

• parsec start TEST-1 --branch <existing-local-branch> creates worktree correctly
• parsec start TEST-2 --branch <remote-only-branch> fetches and creates worktree
• parsec start TEST-3 --branch nonexistent gives clear error
• cargo build && cargo clippy && cargo fmt --check all pass

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