Fix Copilot session-start server outage hint

[wbugitlab1]

Summary

• keep the existing Copilot CLI connect/plugin wiring from origin/main
• add a session-start stderr hint when the AgentMemory server is unreachable
• preserve the hook contract: no stdout on failed fetches and successful hook exit in both telemetry and context-injection paths
• update packaged plugin/scripts/session-start.mjs and hook smoke coverage

Closes #536.

Validation

• Red test: targeted session-start smoke test failed before the fix because stderrWrite was undefined
• corepack pnpm exec vitest run test/hook-source-smoke.test.ts -t "session-start is fire-and-forget by default and writes context only when opted in"
• corepack pnpm run build
• corepack pnpm exec vitest run test/hook-source-smoke.test.ts test/copilot-plugin.test.ts test/build-package-contract.test.ts
• corepack pnpm exec vitest run test/cli-connect.test.ts test/copilot-plugin.test.ts test/onboarding.test.ts test/build-package-contract.test.ts test/hook-source-smoke.test.ts
• corepack pnpm run lint
• corepack pnpm test
• semgrep scan --config p/default --error --metrics=off .
• osv-scanner scan source .
• git diff --cached --check
• gitleaks protect --staged --redact

Notes

• origin/main advanced during the branch prep; merged captured base commit 667f19f542b3959c29a553d17c8571d6cd673ad3 into this branch and reran targeted tests, lint, and full tests.
• The upstream PR's larger Copilot wiring was already present on origin/main; this PR only fills the remaining session-start unreachable-server hint behavior.