refactor(error-handling): use unknown in catch blocks

[github-actions]

Closes #310

Applied changes based on Issue #310.

I’ve enabled strict unknown typing for all catch variables and wired in a small helper to safely stringify them. Changes include:

• tsconfig.json
– Turned on "useUnknownInCatchVariables": true to make all catch params unknown.

• src/utils/error.ts
– New module exporting
• toErrorMessage(error: unknown): string
• toErrorStack(error: unknown): string|undefined

• src/main.ts
– Imported the new helpers and unified the top‐level catch to always use toErrorMessage/toErrorStack.

• src/file/file.ts
– Imported toErrorMessage and protected all unguarded ${error} in its catch blocks.

• src/github/event.ts
– Imported toErrorMessage and wrapped the JSON‐parse error in toErrorMessage.

• src/github/github.ts
– Imported toErrorMessage, and updated several raw core.error(\…${error}`)andthrow new Error(`…${error}`)` sites to use the helper instead.

• src/security/security.ts
– Imported toErrorMessage and switched its two core.warning(\…${error}`)calls totoErrorMessage(error)`.

All existing catch‐blocks that already narrow via instanceof Error remain, but any raw ${error} is now safe.

Next steps:

1. Run npm run format to apply Prettier.
2. Build/compile (npm run build) and verify no TypeScript errors.
3. Run the full test suite (npm test).