docs: surface real-provider E2E testing

[ClaudiaFang]

Summary

• surface the real-provider E2E strategy in the README
• add CONTRIBUTING.md with project testing expectations for contributors
• replace the obsolete temporary E2E note in docs/test-coverage.md with the current layered test model
• add docs/testing/test-scenarios.md as a behavior- and risk-oriented overview
• add docs/testing/test-matrix.md with P0/P1/P2 classification and current coverage by test layer
• document the planned GitHub packaged-plugin black-box P0 suite as a coverage gap and future refactoring guardrail

Why

The repository already has meaningful unit/integration and real-provider E2E coverage, but that quality was difficult to understand from the main documentation. These docs separate four different questions:

• what behaviors and failure modes the project protects
• which scenarios are release-critical (P0) vs important/supporting (P1/P2)
• where the implementation-level tests live
• how the real-provider E2E harness and CI operate

The current real-provider SyncManager suite is documented accurately as gray-box system/E2E coverage because it imports production internals and replaces the Obsidian boundary. A future packaged-plugin black-box suite is explicitly marked Planned, with GitHub as the canonical P0 user journey target. Nothing in this PR claims that black-box coverage already exists.

The wording also intentionally says the real-provider harness supports GitHub, GitLab, and Gitea rather than claiming all three provider legs are currently stable in CI. Current runner-specific limitations remain documented in docs/testing/real-provider-e2e.md.

Validation

• documentation-only change
• reviewed against the current test/real-provider-e2e branch
• test matrix cross-checked against current SyncManager, GitHub, GitLab, and Gitea E2E suites
• planned black-box and compatibility coverage clearly separated from implemented coverage
• final diff reviewed for accidental unrelated changes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud