docs: align documentation with runtime

[QueryPlanner]

Summary

Update repository documentation and the changelog so they describe the current
Blacki runtime. Several documents still refer to Postgres, asyncpg, in-memory
Telegram sessions, an old image name, and OTLP behavior that the current code does
not implement.

Priority

P2

Level of Effort

Small (2-4 hours)

Scope

• Correct README installation, storage, image, session, and observability claims.
• Rewrite the architecture page around the current SQLite and ADK runtime behavior.
• Update development and deployment guides.
• Populate the Unreleased changelog from verified changes since v0.1.0.
• Add automated link and stale-term checks.

Passing Criteria

• Documentation consistently identifies the image as ghcr.io/queryplanner/blacki.
• Storage documentation distinguishes tool data, HTTP ADK sessions, Telegram ADK
  sessions, memory, and artifacts.
• No active setup instructions require Postgres or asyncpg unless presented as an
  optional supported configuration.
• Observability documentation matches the exporter behavior verified by tests.
• The Unreleased changelog accounts for merged changes after v0.1.0.
• An automated documentation check validates links and rejects the known stale
  project/image references.

Sources

• Stale README claims and image
• Stale architecture page
• Stale development guide
• Empty Unreleased changelog