fix(core): preserve command model variants

[altendky]

Issue for this PR

Closes #43474

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Preserves the selected reasoning variant when a command chooses another model that supports the same variant. Explicit command and agent variants still win, while unsupported variants fall back to the destination model's default.

Command resolution now waits for location plugins before consulting command, agent, and model catalogs so variant support is checked against settled configuration.

How did you verify your code works?

• bun test test/session-command.test.ts from packages/core (7 passed)
• bun typecheck from packages/core
• Full monorepo typecheck from the pre-push hook (33 packages passed)
• git diff --cached --check

Screenshots / recordings

Not applicable; this changes command model selection without changing TUI layout.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.