Garden stale planning docs after the ty migration and shipped RFCs

[lbliii]

Context

plan/ is useful, but it now contains many implemented/superseded RFCs and some stale details that conflict with current tooling. That makes it harder for agents and maintainers to know what guidance is active.

Evidence

• plan/ contains about 19k lines of planning material.
• plan/rfc-type-checking-strategy.md is marked implemented but still describes a Pyright -> mypy migration and commands like uv run mypy.
• plan/rfc-type-suppression-reduction.md is marked migrated to ty at the top but still contains mypy-specific implementation commands and override examples.
• plan/rfc-mixin-protocol-typing.md is marked implemented but its key outcome says to remove [[tool.mypy.overrides]], while current pyproject.toml uses [[tool.ty.overrides]].

Proposed Scope

• Add closure notes or supersession pointers to stale RFCs.
• Move active follow-up work into issues/epics and mark old implementation details historical.
• Keep current operational guidance in the closest steward docs, docs/stability-gate.md, and active epics.

Done When

• A maintainer can tell which planning docs are active versus historical.
• Type-checking guidance names ty consistently.
• Any remaining open RFC tasks are linked to GitHub issues or active epics.