fix(setup): reuse existing uv-created venv instead of failing on rerun

[Turkzilla]

Summary

uv venv refuses to write into an existing directory unless --clear is passed. The current setup_environment() in run-server.sh preserves uv-created venvs (by skipping the removal block when the uv_created marker exists), then unconditionally calls uv venv ... against that same directory.

That call fails on every run after the first with:

error: Failed to create virtual environment
  Caused by: A virtual environment already exists at `.pal_venv`. Use `--clear` to replace it

The script then logs uv environment creation failed and falls back to system Python detection, defeating the uv-first design and reinstalling all dependencies through pip on every run.

Fix

Add a fast-path that detects an existing healthy uv-created venv and reuses it (echoing the venv's python, matching the function contract).

If the uv_created marker is present but the Python interpreter is missing or non-executable, the venv is removed and recreated cleanly.

No behavior change for first-time setup or for non-uv venvs.

Test plan

• Existing healthy uv-created venv → fast-path reuses it, no uv venv call
• No venv → original code path runs uv venv --python 3.12
• Marker present but .pal_venv/bin/python missing → venv recreated cleanly
• Non-uv venv (no marker) → existing removal logic preserved
• bash -n run-server.sh syntax check passes

Repro of the bug (before this fix)

$ ./run-server.sh   # first run: succeeds, creates .pal_venv with marker
$ ./run-server.sh   # second run: fails with "already exists", falls back to system Python
Setting up environment with uv...
! uv environment creation failed, falling back to system Python detection
! uv output: Using CPython 3.10.18
Creating virtual environment at: .pal_venv
error: Failed to create virtual environment
  Caused by: A virtual environment already exists at `.pal_venv`. Use `--clear` to replace it

[gemini-code-assist]

Code Review

This pull request updates run-server.sh to allow the reuse of existing uv-created virtual environments, which prevents unnecessary recreations and ensures the uv-first design is maintained. A review comment identifies a potential issue where the Python executable path should be double-quoted to safely handle paths containing spaces.

[Turkzilla]

Failing Lint