feat(core): improve read tool parity

[rekram1-node]

Summary

• support bounded PDF reads as native model file content
• preserve actionable typed Read errors for malformed UTF-8, invalid offsets, and wrong path kinds
• expose V2 Read paths as ACP tool locations while retaining legacy input compatibility
• document intentional lexical symlink permission semantics

Testing

• bun test test/tool-read-filesystem.test.ts test/tool-read.test.ts in packages/core
• bun typecheck in packages/core
• bun test test/acp/tool.test.ts in packages/cli
• bun typecheck in packages/cli
• git diff --check