docs: add Docker Compose self-hosting plan with capability tiers

[jjmata]

Motivation

• Provide a clear, bounded plan to make docker compose up a practical self-hosting entrypoint for the product.
• Capture what the current repository Compose and xtask layers already support and call out the orchestration gaps that block a simple compose-first install.
• Offer an incremental, testable rollout strategy so operators can enable functionality progressively without false promises of full production parity.

Description

• Add a new design doc docs/SELF_HOSTING_COMPOSE_PLAN.md that documents the goal, current state, blockers, and a recommended set of cumulative capability tiers from evaluation → core → search → communications → AI → integrations.
• Propose three canonical Compose layouts (single compose.yml with profiles, base+overlays, and an external-dep override) and prescribe init jobs, gateway behavior, and a small stable .env contract.
• Enumerate the implementation work required: publish distributable images, idempotent init jobs (init-secrets, migrate, init-localstack, init-auth, init-search), single-origin gateway template, persistence/backup guidance, and a security baseline.
• Provide a phased delivery plan with concrete acceptance gates and recommended public wording until the plan reaches Phase 2.

Testing

• git diff --cached --check was run and passed on the staged documentation changes.
• git status --short and a commit were executed to create the change (docs/SELF_HOSTING_COMPOSE_PLAN.md added).
• Attempted docker compose config to validate Compose graph programmatically but the Docker CLI is not available in the execution environment, so that validation was not performed (environment limitation).

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