fix(convo): recognize camelCase filePath/fileName in the shared file-path extractor

[akesling]

Follow-up to #124 (stacked on it — base will auto-retarget to main when #124 merges).

The gap

The shared derive_path file-path extractor (toolpath-convo/src/derive.rs) probed only ["file_path", "path", "filename", "file"] — no camelCase. A provider that leaves file-mutation synthesis to the shared derive (rather than pre-building Turn.file_mutations) therefore dropped a FileWrite tool's artifact when its input used the camelCase convention. opencode's tool inputs use filePath; it dodges the bug only because it pre-builds file_mutations via its own tool_input_file_path helper (which also carries snapshot diffs, so it stays). Any provider without that pre-build — or a future one — silently loses the file from change.

The fix

Add the camelCase siblings of the fields already probed: filePath (for file_path) and fileName (for filename). snake_case is probed first, so no existing input changes which field wins. One line in extract_file_path.

Tests

TDD (both red before the fix): a FileWrite tool whose input has only filePath / only fileName now produces a change entry (previously dropped). Existing snake_case and no-recognized-field tests unchanged. Full workspace build/test/clippy green.

Scope notes

• Strictly additive — only affects newly-derived documents, and only to recover a file that was being dropped. No effect on already-shared documents.
• Deliberately did not fold absolute_path/absolutePath (a distinct gemini-specific field) into the generic list — gemini already handles its own; that's a separate judgment.
• toolpath-convo 0.11.3 (stacked on fix(convo): relativize file change keys against path.base #124's 0.11.2). No path-cli/toolpath-cli bump.

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