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.

[codebuff-team]

Good catch and a clean, minimal fix. This is exactly the kind of gap you'd expect from a squashed export: cli/bunfig.toml references ../test/setup-scm-loader.ts, but the file itself never made it into the mirror, so any test that transitively imports @codebuff/sdk → code-map → .scm files fails at import time with an opaque "Unknown file type" / "Unhandled error between tests" message.

The fix itself is sound: a bun.plugin() onLoad hook that reads .scm files as text and wraps them as a JS module with a default string export, matching what the bundled build already does. That's the standard way to shim non-JS asset imports in bun, and it's scoped tightly to the .scm extension so it won't interfere with anything else.

A few things worth double-checking before this is ported:

• Confirm the private repo's actual setup-scm-loader.ts (if one exists there) matches this shape, or that this is genuinely absent and needs authoring fresh — since this is inferred from bunfig.toml + code-map's import style rather than copied from a known-good source.
• The PR mentions docs/testing.md has a corrupted path (../test/n.ts) referencing the same file; consider filing that as a doc fix too, or folding it into this PR, since it's directly related and one-line.
• No new test asserts the loader behavior itself (e.g., a trivial test importing a .scm fixture) - worth adding so a future refactor of the loader doesn't silently regress.

Overall: small, correctly diagnosed, verified against the actual failure mode. Worth a maintainer's few minutes to confirm and port.

[lab1207]

Thanks for the review! Pushed 9317d5c addressing the actionable ask:

• Loader regression test: cli/src/__tests__/scm-loader.test.ts imports
  the go tree-sitter query .scm directly (a real file shipped by
  code-map, the same import the SDK barrel transitively reaches) and asserts
  it loads as a non-empty string with capture syntax intact. If the preload
  goes missing, the whole file fails to load rather than passing vacuously.
  Verified the discrimination is real: run from the repo root — where
  cli/bunfig.toml's preloads don't apply — the file fails exactly as a
  fresh clone without this PR does.

On the other two asks:

• docs/testing.md ../test/n.ts: I could not reproduce this — the
  current mirror references the correct ../test/setup-scm-loader.ts path
  (in the "global-jsdom / --preload" example near the end), and nothing
  matches test/n.ts anywhere in the repo. That sentence in my PR body was
  an error on my part, sorry for the noise. No doc change needed; happy to
  add one if you're seeing a stale copy.
• Does a private setup-scm-loader.ts already exist? I have no access
  to the private repo, so this is indeed authored fresh — inferred from
  cli/bunfig.toml + code-map's .scm import sites + the bundler's own
  rule (sdk/scripts/build.ts maps .scm → loader 'text' with a default
  string export, which the plugin mirrors).

CLI suite on Windows 11 / bun 1.3.14 after the new commit: 1,584 pass, only
the documented pre-existing exportConversation path-separator failures
(#1290).