build: make optional capabilities installable

[QueryPlanner]

Summary

Align the dependency graph with Blacki's lightweight, cloud-first philosophy.
Memory, sandbox, and provider-specific observability packages are installed in
every environment even when those capabilities are disabled.

Priority

P2

Level of Effort

Medium (4-8 hours)

Scope

• Define a minimal core runtime dependency set.
• Move memory, sandbox, and provider-specific observability packages into explicit
  optional extras or deployment groups.
• Select required capabilities explicitly in Docker builds.
• Make capability discovery fail gracefully when an optional extra is absent.
• Document supported install profiles.

Passing Criteria

• A clean minimal installation can import Blacki and start its core server without
  memory or sandbox packages installed.
• Separate automated test jobs validate minimal, memory, sandbox, and full profiles.
• Tool registration reports disabled optional capabilities without stack traces.
• The production Docker build explicitly selects its required profile.
• uv sync --locked succeeds for every documented profile.
• Dependency-tree output demonstrates that the minimal profile excludes Mem0,
  OpenSandbox, and unused provider-specific exporters.

Sources

• Current unconditional runtime dependencies
• Current capability registry
• Current production dependency installation