docs(env): document SM_MULTI_TENANT and SM_TENANT_HEADER opt-in

[antosubash]

Summary

Final remaining item from the DX hardening plan (docs/plans/2026-04-14-dx-hardening.md — Task 8). The multi-tenant code path and its tests already shipped on main, and docs/framework-conventions.md references the env vars, but .env.example never advertised them — so new deployments had no signpost for the opt-in tenancy flag.

DX hardening plan audit

All 11 implementation tasks from the plan were already landed on main before this branch was created. Verified each on audit:

#	Task	Status
1	Resolve stray validation.ts	Already moved to pages/validation.ts
2	print_diagnostics → stderr	Done + test at test_diagnostics.py:57
3	Dedupe all_module_bases	Done via _register_base + test at test_base.py:38
4	SM_PROJECT_ROOT env override	Done + tests at test_app.py:79,101,106
5	Enforce ModuleMeta at discovery	Done + tests at test_discovery.py:168
6	discover_modules(strict=...) wiring	Done; app_builder passes strict=not settings.is_development
7	Dashboard routing / host landing	Done; view_prefix="/dashboard", landing moved to host/routes.py
8	Opt-in TenantMiddleware	Code done; this PR adds the missing .env.example stubs
9	get_db skip commit when clean	Done + test at test_db_logging.py:76
10	docs/framework-conventions.md	Shipped (373 lines)
11	Rewrite README.md	Shipped (193 lines)

Verification

• make test-py — 977 passed, 6 deselected
• make lint — ruff / ty / biome / tsc all clean; 300-line cap OK
• make doctor — exit 0 (pre-existing SM003 warn on FeatureFlags/Browse.tsx is unrelated)

Test plan

• make lint passes locally
• make test-py passes locally (977 tests)
• make doctor exits 0
• CI green

https://claude.ai/code/session_01EJAM5RUi2pcXPeagmVMK27

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