Stop pinning Go to patch versions, use stable in CI

[rdimitrov]

Summary

• go.mod: go 1.26.2 → go 1.26 — only declare the minimum language version, not a specific patch
• go.mod: bumped toolhive v0.16.0 → v0.17.0 and toolhive-core v0.0.14 → v0.0.15 (these deps were pinning us to go 1.26.2)
• renovate.json: removed the custom regex manager that was bumping the Go patch version (no longer needed), and removed the !golang exclusion from the go modules grouping rule

Similar to stacklok/toolhive-core#82 — avoids noisy Renovate PRs every time a new Go patch drops.

Test plan

• task lint-fix passes locally
• task test passes locally
• go mod tidy does not bump go 1.26 back to a patch version
• CI workflows pick up stable Go and pass

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.86%. Comparing base (e2021fc) to head (73c26fd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #693      +/-   ##
==========================================
+ Coverage   60.84%   60.86%   +0.02%     
==========================================
  Files         103      103              
  Lines        9979     9979              
==========================================
+ Hits         6072     6074       +2     
+ Misses       3377     3376       -1     
+ Partials      530      529       -1     

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