test: make embedding provider tests hermetic

[wbugitlab1]

Summary

• make embedding-provider tests ignore file-backed ~/.agentmemory/.env by mocking config env helpers to read test-controlled process.env only
• document Issue [upstream PR 734] f<!-- -->ix: make hook-project, embedding, and mesh tests hermetic #492 validation and verification in docs/todos/2026-06-18-issue-492-hermetic-tests/

Validation

• Reproduced before fix: hostile HOME/.agentmemory/.env made test/embedding-provider.test.ts fail 15 tests
• Hostile-home repro after fix: 67 tests passed
• corepack pnpm exec vitest run test/hook-project.test.ts test/embedding-provider.test.ts test/mesh.test.ts: 257 tests passed
• corepack pnpm run build: passed
• corepack pnpm run lint: passed
• corepack pnpm run skills:check: passed
• corepack pnpm test: 198 files, 2767 tests passed
• semgrep scan --config p/default --error --metrics=off .: 0 findings
• gitleaks protect --staged --redact: no leaks found

Fixes #492