Add github_path: load a model from a subdirectory of a repo

[stefanoperenzoni]

What

Datasets gain an optional github_path — a subdirectory within the GitHub repo where the model lives. GitHub-loaded models previously required index.malloy at the repo root, which excludes a common layout: a Malloy semantic layer living in a subdirectory of a larger data project (a monorepo with dbt/SQLMesh models, pipelines, etc. alongside the .malloy files).

With this change, a dataset configured with e.g. path=models/malloy resolves index.malloy, all of its imports, malloy-config.json, and the dashboards/ directory under that path.

How

• New datasets.github_path column, NOT NULL DEFAULT '' — empty string means repo root, i.e. existing behavior, byte-for-byte.
• GitHubURLReader gains an optional base path used only when building the Contents-API URL. Cache keys and stored file paths stay model-relative (index.malloy, dashboards/x.malloy), so Malloy import resolution and everything downstream are independent of where the model sits in the repo.
• refreshGitHubModel and the dataset-create introspection prefix the config fetch, the dashboards listing, and dashboard component fetches with the path; provenance records it (github:owner/repo@branch/path).
• The path is settable on dataset create and edit (API + an optional field in both forms) and returned by the dataset GET.
• Manual migration drizzle/manual/0010_github_path.sql (idempotent, per the repo's migration convention); fresh installs get the column from the schema as usual.

Backward compatibility

Dark by default: every existing dataset gets '' and takes exactly the old code path (joinRepoPath('', p) === p). Old API clients that don't send the field are unaffected (zod default). No existing route's behavior changes; the only touched error message is the 404 hint text.

Tests

src/lib/github.test.ts (node:test, fetch stubbed, no network):

• path normalization (slashes/whitespace/null)
• join only when a base is set
• reader with base path: prefixed fetch URL + model-relative cache key + cache hit
• reader without base path: unchanged root behavior (regression guard)

Full preflight is green (engine + CLI typecheck/tests, server lint, next build, hosted integration test).

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