docs(ci-setup): document TOOLR_VENV_LOCATION and the action's in-tree default

[s0undt3ch]

Why

toolr supports an in-repo (tools/.venv) venv via the TOOLR_VENV_LOCATION env key (accepts in-tree / cache), whose stated primary use case is CI. The behaviour is fully documented in docs/project-config.md, but the toolr-ci-setup skill never named it: it described the action "caching the per-repo tools/.venv" and had a cache-miss failure mode, yet never explained why the venv lives in-tree on CI (locally it defaults to the XDG cache dir). The in-tree behaviour read as magic.

What changed

Skill-only edit to skills/toolr-ci-setup/SKILL.md:

• New "Where the venv lives in CI" section:
  • Locally the venv defaults to the volatile per-repo cache dir; the s0undt3ch/ToolR action exports TOOLR_VENV_LOCATION=in-tree to force tools/.venv/, which is exactly what the tools/.venv cache (cache-tools-venv) keys on.
  • Using the action? Do nothing — it sets the env var for you.
  • Bare run: step without the action? Export TOOLR_VENV_LOCATION: in-tree yourself so the venv lands somewhere cacheable (with a YAML snippet).
• Added TOOLR_VENV_LOCATION / "in-tree venv in CI" to the skill's discovery triggers.
• Cross-linked the docs' venv-location reference.

Verification

Skill-only scope (no docs/, Rust, or Python touched):

• cargo xtask build-skill-refs --check ✅
• prek run on the changed file ✅

[s0undt3ch]

This change is part of the following stack:

• docs(ci-setup): document TOOLR_VENV_LOCATION and the action's in-tree default #375 ◀

_{Change managed by git-spice.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.8%. Comparing base (0f3b3a8) to head (af2d15a).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #375     +/-   ##
=======================================
- Coverage   93.8%   93.8%   -0.1%     
=======================================
  Files         96      96             
  Lines      15810   15939    +129     
  Branches     118     118             
=======================================
+ Hits       14838   14958    +120     
- Misses       962     971      +9     
  Partials      10      10             

Flag	Coverage Δ
3.11	93.8% <ø> (-0.1%)	⬇️
3.12	93.8% <ø> (-0.1%)	⬇️
3.13	93.8% <ø> (-0.1%)	⬇️
3.14	93.8% <ø> (-0.1%)	⬇️
macos-latest	93.8% <ø> (ø)
ubuntu-latest	93.8% <ø> (ø)
windows-latest	90.4% <ø> (+61.7%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
toolr-core	94.3% <ø> (-0.1%)	⬇️
toolr	92.8% <ø> (+0.1%)	⬆️
toolr-py	96.0% <ø> (ø)
xtask	90.3% <ø> (ø)
see 8 files with indirect coverage changes