[speed up ucode] Skip preflight for configured agent launches

[lilly-luo]

Configured agent launches now trust their persisted preflight state instead of repeating Databricks auth, AI Gateway validation, and model discovery. First-run and auto-configured launches continue to run the full preflight.\n\nTests: PYTHONPATH=src pytest -q tests/test_cli.py; ruff check .; ruff format --check src tests

wall clock time with this change is 2s faster than without (4.4 vs 6.3)

$ time ucode claude        
✔ Databricks auth already available for https://dbc-a5d4177a-49dc.cloud.databricks.com
✔ Unity AI Gateway detected

╭────────────────────────╮
│ ucode with Claude Code │
╰────────────────────────╯
  Model: system.ai.claude-opus-4-8
  Smart routing: enabled
✔ Starting Claude Code
ucode claude  4.15s user 1.23s system 85% cpu 6.327 total
# lilly.luo at ip-10-93-38-229 in ~/worktrees/ucode-demo (git:lilly/improve-launch-time) [0:01:21]
$ time uv run ucode claude

╭────────────────────────╮
│ ucode with Claude Code │
╰────────────────────────╯
  Model: system.ai.claude-opus-4-8
  Smart routing: enabled
✔ Starting Claude Code
uv run ucode claude  4.10s user 1.15s system 119% cpu 4.407 total

test

• ucode claude
• ucode codex (it's 2s faster)
• ucode claude --workspace xxxx
• delete all my ~/.ucode stuff and ucode claude -should be slower to configure but work