feat: expose subagent tool activity as persistent CLI lines

[0xbyt4]

Summary

Fixes #169 -- subagent actions are now visible to the user during delegation.

Before: Subagent tool calls only updated the delegate spinner text, overwriting each other. The user saw only the last tool call and all child stdout was suppressed via redirect_stdout(devnull).

After: Each subagent tool call prints a persistent indented line below the spinner:

(o´▽`o) ⠋ 🔀 researching login bug
  ┊  ↳ [sub-1] terminal "find . -name '*.py'"
  ┊  ↳ [sub-1] read_file "src/auth/login.py"
  ┊  ↳ [sub-1] search_files "password validation"
  ┊ 📖 read      src/auth/login.py  0.3s
  ┊ 🔎 grep      password validation  1.2s
┊ 🔀 delegate  researching login bug  12.3s

Changes (single file: tools/delegate_tool.py)

• _child_progress callback now prints persistent ┊ ↳ [sub-N] tool "preview" lines instead of updating the spinner text
• Removed redirect_stdout(devnull) wrapper -- child runs with quiet_mode=True which already limits output to formatted tool lines
• Removed sys.stdout/sys.stderr save/restore workaround (no longer needed without redirect)
• Cleaned up unused imports (contextlib, io, sys)

Why this is safe

• quiet_mode=True on child agents already suppresses raw model output -- only cute tool messages are printed
• prompt_toolkit.patch_stdout (active in CLI) handles concurrent writes from background threads -- same pattern already used by batch mode completion lines (line 328)
• Gateway progress forwarding (parent_progress_cb) is unchanged

[teknium1]

Sorry I implemnented this one: #186