Remove migrate command

[Copilot]

Removes the rhiza migrate command which was used to transition projects from the legacy .github/rhiza/ structure to the .rhiza/ folder structure.

Changes

• CLI: Removed migrate command registration and import from cli.py
• Implementation: Deleted src/rhiza/commands/migrate.py (200 lines)
• Tests: Deleted tests/test_commands/test_migrate.py (424 lines)
• Documentation: Removed migrate sections from README.md and CLI.md
• References: Updated validate.py and uninstall.py to remove suggestions to run rhiza migrate

Verification

$ rhiza --help
# migrate command no longer listed

$ rhiza migrate
# Error: No such command 'migrate'

All 140 existing tests pass.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• astral.sh
  • Triggering command: /usr/bin/curl curl -LsSf REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Remove the migrate command</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Remove the migrate command #205

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