fix(modern-python): suggest exact uv run python so shim advice works outside projects

[Ninja3047]

Bug

As reported in #195: the python/python3 shim's suggested command can be a dead end. The shim echoes back whichever name was invoked (uv run $cmd ...), and for python3 that advice fails on any machine with no uv-managed interpreters (fresh CI runners, devcontainers, stock Ubuntu/Debian): outside a project, uv resolves the python3 command through an ordinary PATH lookup, which hits the shim again and errors with the same suggestion — an infinite advice loop for the agent.

Reproduced on a stock ghcr.io/astral-sh/uv:debian container (apt python3, zero managed Pythons, no env overrides, no project):

$ uv run python3 script.py
ERROR: Use `uv run python3 script.py` instead of `python3 script.py`

Fix

uv special-cases the exact command name python (target.eq_ignore_ascii_case("python") → RunCommand::Python in crates/uv/src/commands/project/run.rs, present since at least uv 0.4.0): uv run python ... executes uv's resolved interpreter directly with no PATH lookup, so it can never resolve back to the shim.

The shim now always suggests the exact-python spelling — uv run python ... / uv run python -m ... — regardless of whether python or python3 was intercepted (the "instead of" half still quotes what was actually run). Verified on the same stock Debian container and on macOS that the suggested form succeeds for script, -c, -m module, and REPL invocations, inside and outside projects.

This is an alternative to the passthrough approach in #195: the shim stays a pure print-and-exit blocker (no UV env detection, PATH walking, or exec-loop guards), keeps working when UV leaks in from ancestors like uv run claude or Agent SDK apps launched via uv run, and lowers the uv floor from 0.6.0 to 0.4.0. Known non-goal: nested tooling under uv run that hardcodes python3 (e.g. a Makefile) still gets blocked with advice it can't follow mechanically — acceptable for an agent-facing nudge, and #195's passthrough remains a possible follow-up if that case matters.

Also updates the README/setup-shims.sh rationale (the old "uv run prepends its venv bin/" claim only holds inside a project or with managed interpreters installed).

Tests

• python-shim.bats: two new tests assert the suggestion uses exact uv run python (never python3) when invoked via the python3 symlink, for both script and -m forms; the existing symlink test now checks the "instead of" side. All 10 pass (bats 1.12), full shims suite 35/35 green.
• shellcheck -x (0.11.0) and shfmt -i 2 -ci (3.13.1) clean.
• check_claude_loadability.py / check_codex_loadability.py pass.
• Version bumped 1.5.2 → 1.5.3 in plugin.json and root marketplace.json.

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