feat(linters): add contextcancelnotdeferred analyzer

[Copilot]

Adds a new custom Go analyzer to catch context.WithCancel/WithTimeout/WithDeadline cancel funcs that are invoked directly instead of deferred. This targets context/goroutine leak risk when early returns or panics occur between context creation and cancel invocation.

• Analyzer implementation

  • Added pkg/linters/contextcancelnotdeferred/contextcancelnotdeferred.go.
  • Tracks cancel funcs returned from context.WithCancel, context.WithTimeout, and context.WithDeadline.
  • Reports when cancel() is called directly and no corresponding defer cancel() exists in the same function.
  • Mirrors existing linter conventions:
    • skips test files via filecheck.IsTestFile
    • does not descend into function literals (closure scope isolation)
    • handles reassignment/shadowing via types.Object-based tracking

• Linter registration

  • Registered contextcancelnotdeferred.Analyzer in cmd/linters/main.go so it runs in the multichecker binary.

• Analyzer tests and fixtures

  • Added contextcancelnotdeferred_test.go with analysistest.
  • Added fixtures under testdata/src/contextcancelnotdeferred/ covering:
    • flagged direct cancel calls
    • valid deferred cancel patterns
    • closure behavior
    • reassignment and blank identifier cases

Example pattern now flagged:

ctx, cancel := context.WithTimeout(parent, 5*time.Second)
result, err := doWork(ctx)
cancel() // should be defer cancel()