refactor(core): tag read outputs

[rekram1-node]

Summary

• add explicit file, text-page, and list-page discriminators to Read outputs
• keep the shared FileSystem.Content schema unchanged
• replace structural encoding checks with direct output-type narrowing

Testing

• bun test test/tool-read-filesystem.test.ts test/tool-read.test.ts
• bun test test/session-instructions.test.ts
• bun typecheck
• git diff --check