fix(search_code): render prose snippets + make context work + path-is-file guard

Address bugs found by an end-to-end test of the tool:

- HIGH: prose/text files (.md/.html/etc.) came back as bare headers with no
  snippet. cs's default 'auto' snippet mode emits a single 'content' string
  (no 'lines[]') for prose, which the renderer skipped. Force
  --snippet-mode=lines by default so every file type returns a lines[] window
  that renders. Also add a defensive 'content'-shape fallback in formatResults
  (+ widen the CsResult type) so a content result is never shown blank.
- HIGH: the 'context' parameter was a no-op — cs ignores -C except in grep
  snippet mode. When context is supplied, switch to --snippet-mode=grep so -C
  actually widens the per-match window (verified 2 -> 26 lines); default
  (no context) keeps the richer lines window for code.
- LOW: a 'path' pointing at a file (not a dir) silently returned 'No matches
  found' (cs --dir <file> => null). Now stat the path and return an
  explanatory error (file vs nonexistent), pointing at read_file for a file.
- MEDIUM/doc: clarify snippet_length (prose-mostly) and context descriptions.

Tests: +5 (prose rendering live + stubbed content-shape; context widening;
path-is-file; path-nonexistent). Full suite 603 pass, biome + tsc green.

Note: the EACCES spill failure seen in testing is pre-existing platform
infra (truncate.ts SPILL_ROOT, shared by all tools), not part of this tool.

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