Remove the stale PLAN.md implementation plan #167

Problem

PLAN.md is the original pre-implementation, phase-by-phase plan. It describes the project as it existed before any code landed:

• Root layout my-libsql-driver/ instead of the five real modules in the root pom.xml.
• Package org.libsql instead of com.github.magnusp.libsql.
• URL form jdbc:libsql://host:port?authToken=...&tls=true instead of the implemented jdbc:libsql:http(s)://host.
• Optional WebSocket transport and Protobuf codec phases that were never the chosen path.

Everything the plan promised is now implemented, and the current architecture is documented in AGENTS.md §2 and the module READMEs. The plan is no longer a plan and misleads anyone who reads it.

Proposed change

Delete PLAN.md. Fold any still-relevant architecture summary into AGENTS.md §2 (it is already there), not into README.md.

Acceptance criteria

• PLAN.md is removed from the repository root.
• No other file links to PLAN.md (grep to confirm).
• AGENTS.md §2 still describes all five modules, including libsql-conformance.