fix: preserve explicitly selected named provider choices

[Brian Krabach (bkrabach)]

Summary

Preserve an explicitly selected named provider configuration when the selected provider is represented by one unnamed mount in the prepared bundle.

This is an independent CLI correctness fix. It does not change provider defaults or require any new module/provider API. It should land before provider preset-default changes so a user who explicitly selects a saved provider cannot have that choice lost when the root/provider defaults are promoted.

Behavior

• --provider anthropic recognizes the saved provider with id: anthropic and module: provider-anthropic.
• The saved selection replaces the single matching unnamed provider-anthropic mount; it does not append a duplicate mount.
• The saved selection's model/configuration is retained (tested with saved claude-sonnet-5 versus the unnamed root preset claude-sonnet-5-5).
• An exact already-mounted provider ID remains authoritative.
• Multiple matching unnamed mounts fail clearly as ambiguous rather than selecting one silently.
• With no provider selection, existing behavior is unchanged.

Verification

Scoped to the published Core 2.0.1 test environment:

• Full CLI suite: 2471 passed, 1 skipped, 13 deselected, 1 xfailed.
• Focused provider-selection tests: 64 passed.
• Independent read-only code review: PASS (not a test-suite count).
• Real macOS and Linux CLI runs verified fresh selection of the 5.5 preset and preservation of a saved id: anthropic selection using claude-sonnet-5.
• The exact mounted-provider ID and ambiguous-mount cases are covered by regression tests.
• runtime/config.py is unchanged.

Scope and sequencing

This PR is limited to the CLI's provider-selection path, its regression tests, and the related README clarification. It does not modify the provider default itself, module/provider APIs, computer-use behavior, downstream repositories, or any provider refs.

Review notes

• No breaking changes intended.
• No new public content class or provisioning change.
• Please review for the CLI behavior and merge ordering relative to the provider preset-default promotion.