fix(copilot): use --yolo to grant all permissions in non-interactive mode (#2298)

* fix(copilot): use --yolo to grant all permissions in non-interactive mode

The Copilot CLI's --allow-all-tools flag only covers tool execution
permissions but does not grant path or URL access. When the Copilot
agent autonomously runs shell commands (e.g. npm run build) during
workflow execution, the CLI blocks path access and cannot prompt for
approval in non-interactive mode, producing:

  Permission denied and could not request permission from user

Replace --allow-all-tools with --yolo (equivalent to --allow-all-tools
--allow-all-paths --allow-all-urls) to grant all three permission types.

Rename the opt-out env var from SPECKIT_ALLOW_ALL_TOOLS to
SPECKIT_COPILOT_ALLOW_ALL to match the formal --allow-all alias and
scope it to the Copilot integration.

Fixes #2294

* review: deprecate SPECKIT_ALLOW_ALL_TOOLS, rename to SPECKIT_COPILOT_ALLOW_ALL_TOOLS

Address Copilot review feedback:

- Honour the old SPECKIT_ALLOW_ALL_TOOLS env var as a fallback with a
  DeprecationWarning so existing opt-outs are not silently ignored.
- Rename the new canonical env var to SPECKIT_COPILOT_ALLOW_ALL_TOOLS.
- New var takes precedence when both are set.
- Use monkeypatch in tests to avoid flakiness from ambient env vars.
- Add tests for deprecation warning, precedence, and opt-out paths.

* review: use UserWarning instead of DeprecationWarning

DeprecationWarning is suppressed by default in Python, so users relying
on the old SPECKIT_ALLOW_ALL_TOOLS env var would never see the
deprecation notice during normal CLI runs. Switch to UserWarning which
is always shown. Update test to also assert the warning category.