fix: resolve schema paths relative to project root

[tejaskash]

Summary

• Schema file paths (OpenAPI/Smithy) are now resolved relative to the project root (parent of agentcore/) instead of inside agentcore/
• This allows specs/ directory to live alongside app/ and agentcore/ at the top level:

  project/
  ├── app/
  ├── agentcore/
  │   └── mcp.json
  └── specs/
      └── petstore.json
  

• Updated path resolution in validate.ts to use dirname(configRoot)
• Updated CLI --schema help text and TUI prompts to say "relative to project root"
• Removed agentcore/ prefix from TUI confirm review display
• Updated integration tests to write schema files at project root level

Test plan

• All 2052 unit tests pass
• All 22 integration tests pass (add-remove-gateway)
• ESLint, prettier, typecheck all pass (pre-commit hooks)

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	42.58%	3750 / 8805
🔵	Statements	42.18%	3956 / 9378
🔵	Functions	44.28%	740 / 1671
🔵	Branches	44.38%	2455 / 5531

Generated in workflow #917 for commit 1d5fde7 by the Vitest Coverage Report Action