feat(adapters): parse OpenAI-compatible prompts

[kev1n77]

Summary

• add an adapter registry with confidence ordering, invalid-confidence rejection, adapter error/panic isolation, Prompt IR validation, structured partial issues, and Raw fallback
• add an OpenAI-compatible adapter for Responses, Chat Completions, and Completions requests
• map system/developer/user/assistant/tool messages, multimodal parts, function definitions, tool calls/results, generation options, and streaming request flags with JSON-pointer evidence
• replace the desktop demo's handwritten Prompt IR fixture with the real Capture Policy -> adapter -> Prompt IR -> SQLCipher path
• update capture documentation and mark PAR-002/003 complete

Contract evidence

• checked capture/golden pairs cover Responses and Chat streaming requests with tools and multimodal content
• completions arrays remain distinct user messages
• tool use IDs match subsequent tool result references
• unknown content is preserved as PromptPart::Unknown with a partial issue
• invalid JSON and unsupported providers fall back to Raw
• rejecting, panicking, and invalid-output adapters do not prevent a lower-confidence valid candidate from succeeding

Verification

• cargo fmt --all -- --check
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --manifest-path apps/desktop/src-tauri/Cargo.toml -- --check
• cargo test --manifest-path apps/desktop/src-tauri/Cargo.toml
• cargo clippy --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets -- -D warnings
• npm run desktop:check
• npm run desktop:build