feat(core): improve shell tool guidance

[rekram1-node]

Summary

• improve Shell parameter and model-facing descriptions
• include the active operating system and resolved shell in each model request
• remove the maximum timeout while retaining the two-minute foreground default and explicit unlimited mode
• remove stale Shell TODOs for Windows invocation and file-backed output capture

Testing

• bun test test/tool-shell.test.ts test/shell.test.ts (packages/core)
• bun typecheck (packages/core)
• git diff --check origin/v2...HEAD