docs: update AGENTS.md with testing guidelines and architecture

[BYK]

Summary

Updates AGENTS.md to reflect the current state of the project, with a focus on emphasizing property-based and model-based testing using fast-check.

Key Changes

• Testing Section Rewrite: Establish property-based and model-based testing as the preferred approach over traditional unit tests

  • Add testing hierarchy: model-based > property-based > unit tests
  • Document test file naming conventions (*.property.test.ts, *.model-based.test.ts)
  • Add examples for both testing patterns
  • Document shared test helpers (createIsolatedDbContext, DEFAULT_NUM_RUNS)

• Architecture Updates: Reflect all new files added to the codebase

  • src/lib/db/: utils.ts, regions.ts, migration.ts, user.ts, version-check.ts, etc.
  • src/lib/dsn/: project-root.ts, code-scanner.ts, fs-utils.ts, errors.ts
  • src/lib/: region.ts, telemetry.ts, sentry-urls.ts, upgrade.ts, etc.

• New Key Features: Document recent additions

  • Multi-region support with fan-out to regional APIs
  • Project root detection using VCS/language/build markers
  • Directory name inference for fallback project matching

• Error Handling: Add new error types (SeerError, UpgradeError)

• New Patterns: Document SQL upsert() helper utilities

The implementation plan is available as a git note on the commit.

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

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New Features ✨

Dsn

• Infer project from directory name when DSN detection fails by BYK in #178
• Add project root detection for automatic DSN discovery by BYK in #159

Other

• (issue) Replace --org/--project flags with /ID syntax by BYK in #161
• (lib) Add anyTrue helper for parallel-with-early-exit pattern by BYK in #174
• (telemetry) Add withTracing helper to reduce Sentry span boilerplate by BYK in #172

Bug Fixes 🐛

• (types) Align schema types with Sentry API by betegon in #169
• ParseIssueArg now checks slashes before dashes by BYK in #177
• Address bugbot review comments on dsn-cache model-based tests by BYK in #176
• Added nullable in substatus's zod validation by MathurAditya724 in #157

Documentation 📚

• Update AGENTS.md with testing guidelines and architecture by BYK in #190

Internal Changes 🔧

• (upgrade) Use centralized user-agent for GitHub API requests by BYK in #173

Other

• test: add coverage for UpgradeError and SeerError classes by BYK in #188
• test: add property tests for sentry-urls.ts (Phase 3) by BYK in #186
• test: simplify issue-id tests covered by property tests by BYK in #183
• test: simplify alias and arg-parsing tests covered by property tests by BYK in #182
• test: add property tests for API command and human formatters by BYK in #181
• test: remove redundant DB tests covered by model-based tests by BYK in #180
• test: add property tests for async utilities (Phase 4) by BYK in #179
• test: add model-based tests for DSN and project cache by BYK in #171
• test: add model-based and property-based testing with fast-check by BYK in #166

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