Cap terminal output backlog

[luckeyfaraday]

Summary

• cap pending PTY output in the Electron main process and PTY host
• close overloaded terminal SSE streams instead of buffering indefinitely
• clear terminal output buffers/listeners when panes exit or are killed

Verification

• npm run build:electron
• node --test tests/terminal-buffer.test.mjs
• npm run test:electron
• npm run dist

[luckeyfaraday]

Closing as redundant: PR #169 (fix/terminal-output-flow-control) is a strict superset of this branch — it contains both commits here (aafdaac Cap terminal output backlog, a5f9b64 Bound terminal renderer output flow) verbatim, plus ee83d09 (Bound agent session transcript scans). Every file this PR touches is touched identically in #169. Consolidating the terminal output flow work into #169.