feat: add --pull-default flag to par start and workspace start

[userhas404d]

Summary

Closes #23

Adds --pull-default support to both par start (single-repo) and par workspace start (multi-repo) commands.

When --pull-default is used, the default branch for each repository is detected via origin/HEAD, pulled with --ff-only, and used as the base for new worktree branches. An explicit --base still takes precedence.

Changes

Single-repo par start --pull-default

• Added pull_default: bool parameter to core.start_session()
• Added --pull-default CLI flag to start / create commands
• When set: detects default branch, pulls it, uses it as worktree base
• 4 new tests covering CLI forwarding and core behavior

Multi-repo par workspace start --pull-default (existing)

• Already implemented — included here for completeness of the upstream PR

README

• Updated quick start examples
• Updated detailed session docs
• Updated "Keeping branches up-to-date" section to cover both single-repo and workspace usage

Usage

# Single-repo: pull default branch before creating worktree
par start feature-auth --pull-default

# Single-repo: --base still takes precedence
par start feature-auth --pull-default --base develop

# Multi-repo workspace (existing behavior)
par workspace start feature-auth --repos frontend,backend --pull-default

Testing

All 48 tests pass, including 4 new tests:

• test_cli_start_forwards_pull_default
• test_cli_start_pull_default_off_by_default
• test_start_session_pull_default_uses_default_branch_as_base
• test_start_session_pull_default_with_explicit_base