Simplify project context/auth flow for read-only commands + persist project_id

[Parker Henderson (parkerhendo)]

1. bt switch now persists project_id in config (alongside org and project).
2. Read-only commands (list/view paths) avoid full reauthentication when a cached
   project_id exists.
3. Spinner display delay increased from 300ms to 500ms.

[github-actions]

Latest downloadable build artifacts for this PR commit 8a5ddc0912ca:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/23156797949
• Download all artifacts (GitHub CLI): gh run download 23156797949 --repo braintrustdata/bt
• Installers are published from main automatically. To publish one for a PR branch, run release-canary manually via workflow_dispatch.

Available artifact names

• ``artifacts-build-global
• ``artifacts-build-local-x86_64-apple-darwin
• ``artifacts-build-local-x86_64-pc-windows-msvc
• ``artifacts-build-local-x86_64-unknown-linux-gnu
• ``artifacts-build-local-aarch64-apple-darwin
• ``artifacts-build-local-aarch64-unknown-linux-gnu
• ``artifacts-plan-dist-manifest
• ``cargo-dist-cache

[Olmo Maldonado (ibolmo)]

happy to try to review (rust is not yet my strong suit), but at a high level this pr could use some tests. i'll come back around today, but let me know if you'd like to proceed without tests Parker Henderson (@parkerhendo)

[Parker Henderson (parkerhendo)]

happy to try to review (rust is not yet my strong suit), but at a high level this pr could use some tests. i'll come back around today, but let me know if you'd like to proceed without tests Parker Henderson (Parker Henderson (@parkerhendo))

Olmo Maldonado (@ibolmo) I'll go back through and add some tests for it.

[Parker Henderson (parkerhendo)]

Olmo Maldonado (@ibolmo) added some tests for the new paths.