ci(plugins): sherpa-onnx and CTranslate2 caches miss include headers

[staging-devin-ai-integration]

Problem

The plugin CI workflow caches sherpa-onnx and CTranslate2 libraries but not their include headers.

sherpa-onnx (plugins.yml)

• Cache path: /usr/local/lib/libsherpa*
• Install step also copies: /usr/local/include/ (headers from the sherpa-onnx archive)
• On cache hit, the lib files are restored but headers are not.

CTranslate2 (plugins.yml)

• Cache path: /usr/local/lib/libctranslate2*
• Install step also installs: headers via sudo make install
• On cache hit, the lib files are restored but headers are not.

Current impact

None — the Rust plugins use FFI via build.rs with cargo:rustc-link-lib / cargo:rustc-link-search directives (no bindgen against headers). Clippy succeeds on cache hits even without includes.

Future risk

If any plugin starts using bindgen or cc to compile against sherpa-onnx or CTranslate2 headers, cache hits would fail silently.

Suggested fix

Expand the cache paths to include headers:

# sherpa-onnx
path: |
  /usr/local/lib/libsherpa*
  /usr/local/include/sherpa-onnx

# CTranslate2
path: |
  /usr/local/lib/libctranslate2*
  /usr/local/include/ctranslate2

Or cache the entire install prefix.

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Surfaced during CI consolidation in #295.