adds slack alert for failed toolhive releases

[ChrisJBurns]

ToolHive release builds fail often enough to the point we want to know exactly when they happen so we can stop making assumptions that they succeeded.

Example:

  ┌─────────────────────────────────────────────────────────┐
  │  🚨 ToolHive Release Failed                             │
  ├─────────────────────────────────────────────────────────┤
  │  Version:              Triggered by:                    │
  │  v0.5.0                chrisburns                       │
  ├─────────────────────────────────────────────────────────┤
  │  Workflow Run:                                          │
  │  View Failed Run  ← (clickable link)                    │
  ├─────────────────────────────────────────────────────────┤
  │  Repository: stacklok/toolhive | Commit: abc123def...   │
  └─────────────────────────────────────────────────────────┘

  The "View Failed Run" link would take you directly to the GitHub Actions run page (e.g., https://github.com/stacklok/toolhive/actions/runs/12345678) where you can investigate which job failed and view the logs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.87%. Comparing base (e00a4fc) to head (13a49ff).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3086   +/-   ##
=======================================
  Coverage   56.87%   56.87%           
=======================================
  Files         335      335           
  Lines       33572    33572           
=======================================
  Hits        19093    19093           
  Misses      12890    12890           
  Partials     1589     1589           

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