fix(serve): degrade startup banner to ASCII on non-UTF-8 consoles (#522)

[padak]

Summary

Fixes #522 — kbagent serve --ui crashed on startup on Windows with a non-UTF-8
console codepage (e.g. cp1250 on Czech/Polish/Hungarian Windows 11).

The startup banner in commands/serve.py writes box-drawing glyphs (├─ / └─)
via sys.stdout.write. On a legacy single-byte console those glyphs cannot be
encoded, so the write raised UnicodeEncodeError before uvicorn.run was
reached — the server never bound the port. Both the --ui and API-only banner
branches were affected.

Fix

• Transliterate the box-drawing glyphs to ASCII (|- / `-) when
  sys.stdout.encoding can't represent them. This mirrors the UTF-8/ASCII
  fallback install.sh
  already uses for its own banner (Unicode only under a UTF-8 locale, ASCII
  otherwise).
• A belt-and-braces try/except UnicodeEncodeError around the write re-emits
  lossily, so a purely cosmetic banner can never abort server startup.
• Modern UTF-8 terminals are unaffected — the glyphs are kept verbatim.

The banner logic is split into two small helpers to match the existing style in
this module (cf. _default_conversation_id):

• _encode_safe(text, encoding) — pure transliteration decision, fully unit-testable.
• _write_banner(text) — the I/O wrapper with the fallback.

Testing

New tests/test_serve_banner_encoding.py (19 tests):

• A faithful cp1250 stdout fake whose write() raises UnicodeEncodeError on
  the glyphs exactly like a real Windows console — with a fidelity test proving
  the fake reproduces the crash, so the passing case isn't vacuous.
• Branch coverage for _encode_safe (UTF-8 keeps glyphs; cp1250/cp1252/latin-1/
  ascii transliterate; None encoding; unknown-codec LookupError).
• _write_banner doesn't crash on cp1250, preserves Unicode on UTF-8, and the
  belt-and-braces path survives a stream that lies about its encoding.
• An AST-based regression guard: it parses serve_command, extracts the live
  banner = (...) literals, and asserts every non-ASCII glyph they emit has an
  entry in _BANNER_ASCII_FALLBACK — so a future glyph added to the banner
  without a fallback fails CI rather than shipping another crash.

All 69 serve-related tests pass; ruff check, ruff format --check, ty check,
and make changelog-check are green. No command signature changed, so no
plugin/agent-doc sync is required.

Reproduction (before this fix)

set PYTHONUTF8=0
kbagent serve --ui
# UnicodeEncodeError: 'charmap' codec can't encode characters ... cp1250.py

The documented workaround (set PYTHONUTF8=1) is no longer needed.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.