File-source docs use `path:` but the real config key is `file:` (csv/json/markdown)

[adnaan]

File-source docs use path: but the real key is file: (csv / json / markdown)

Surfaced during review of #310. The M5 PR fixed this exact class of bug for docs/sources/wasm.md (which really does use path:), and the reviewer noted csv.md has the mirror problem. It's actually all three file-source docs:

Doc	Documents	Source reads	Result of following the docs
docs/sources/csv.md	path:	cfg.File (file:) — internal/source/source.go:242	csv source: file is required
docs/sources/json.md	path:	cfg.File (file:) — source.go:240	load failure
docs/sources/markdown.md	path:	cfg.File (file:) — source.go:245	load failure

(wasm correctly uses path: → cfg.Path; that's the one exception, and it's already correct after #310.)

Fix

• Change the path: examples to file: in docs/sources/{csv,json,markdown}.md, matching the working examples/lvt-source-file-test/ (json/csv) and the config.SourceConfig.File yaml:"file" field.
• Optional: a small test asserting each docs/sources/*.md config example uses a key the corresponding source actually reads (this class of doc-vs-reality drift keeps recurring — wasm.md, now these).

Kept out of #310 to hold that PR to its M5 scope (save & share). Low-risk, docs-only.

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https://claude.ai/code/session_018M9pJSPmG6i1D8s6rpEV4h