fix(terminal): select the Windows shell from ComSpec

[lzcer (lzcer)]

Summary

• preserve an explicit SHELL for users who configure one
• select ComSpec/COMSPEC on Windows, with cmd.exe as the final fallback
• keep the existing /bin/bash fallback on non-Windows platforms
• keep shell selection local to the terminal module instead of mutating the process environment

Problem

ThinkRail can be launched from a Windows GUI environment where SHELL is not defined. The terminal manager currently falls back to /bin/bash, which is not a valid default Windows executable, so bun-pty cannot spawn the terminal and the tab reports could not start a shell.

Approach

The terminal module now resolves the PTY executable from the environment and platform. An explicitly configured SHELL still wins on every platform. Windows otherwise follows its native ComSpec convention, while Unix-like platforms retain the existing bash fallback. This avoids inventing a global SHELL value that would also affect the in-process agent and unrelated child processes.

Validation

• bun test packages/server/src/terminal/shellArgs.test.ts (5 pass, 0 fail)
• real Windows bun-pty spawn using C:\Windows\System32\cmd.exe
• bun run check:deps
• bun run check:boundaries
• bun run check:seams
• bun run lint (passes with 6 existing warnings)
• bun run typecheck

The complete unit and no-agent E2E suites were also attempted on Windows. Existing fixture/platform assumptions fail before this terminal path is exercised, including spawnSync mv ENOENT, missing project fixture rows, JetBrains Central remaining absent, temporary-directory EBUSY, and POSIX sleep usage.

[danyaberezun]

Approved — ready to merge.