Add issue-flow update and safer re-init messaging

[jepegit]

Closes #4

Summary

• Add issue-flow update to refresh packaged Cursor slash commands, rules, and docs/cursor-issue-workflow.md from the installed package, and to recreate any missing .issueflows/ subdirectories.
• Refactor manifest writes so init and update share one code path; init without --force still skips existing manifest outputs so issue markdown under .issueflows/ is never touched by these commands.
• When init sees an existing scaffold and --force is not used, print a short note pointing users at issue-flow update versus init --force.
• README and developing docs describe when to use init, update, and --force.
• Tests cover overwrite behavior, preservation of issue files, and recreation of a removed subfolder.

How to test

uv sync
uv run pytest
uv run issue-flow update --help

Made with Cursor