fix(core): unify model-visible shell output previews

[opencode-agent]

Summary

• reuse one shell-output preview for model tool calls, background completions, and direct session shell commands
• apply the same configured byte and line limits, tail selection, truncation marker, and full-output file reference on every path
• cover oversized direct shell output and configured line limits with regression tests

Verification

• SHELL=/bin/bash bun test test/session-create.test.ts test/tool-shell.test.ts (66 passing)
• bun typecheck

Requested by: @rekram1-node (Aiden via Slack)