Add missing test/setup-scm-loader.ts preload for .scm imports

[lab1207]

Problem

cli/bunfig.toml lists three test preloads:

preload = [
  "../sdk/test/setup-env.ts",
  "../test/setup-scm-loader.ts",
  "./test/setup-agents-artifact.ts",
]

But test/setup-scm-loader.ts does not exist in the public mirror. Any test that
reaches @codebuff/sdk (which re-exports code-map, which imports .scm
tree-sitter query files) dies at import time with "Unknown file type" — which bun
reports as "Unhandled error between tests", so a fresh clone shows a wall of
failing test files with no obvious cause. (The same file is referenced in
docs/testing.md, there under a corrupted path ../test/n.ts.)

Fix

One new file: a bun plugin() that loads .scm imports as a default-exported
string — the same thing the bundled build does. Registered via the preload that
bunfig.toml already declares, so no config change is needed.

Verification

cd cli && bun test src/commands src/utils/__tests__ src/state:

• 1,576 pass with this change
• 13 failures remain, all pre-existing Windows path-separator issues in
  export-conversation.test.ts (expects /project/..., gets \project\...) —
  present on main without this change, happy to file a separate issue for those

Tested on Windows 11 / bun 1.3.14.