Reimplement GoString for keys with functional option format

[mpyw]

Summary

• Remove GoString from Inspection and BoolInspection
• Reimplement GoString for key[V] and boolKey to output valid Go expressions with functional option format
  • feature.New[string](feature.WithName("key-name"))
  • feature.NewBool(feature.WithName("flag-name"))
• Update README with background on Go official proposal (proposal: context: add generic Key type golang/go#49189) and design decisions (Sealed Interface pattern, empty struct optimization avoidance)

Test plan

• go test ./... passes
• golangci-lint run passes
• GoString output is validated to compile with go/types

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (375ee78) to head (0b85ee5).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #3   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         3    +1     
  Lines          113       108    -5     
=========================================
- Hits           113       108    -5     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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