fix(pi): let agent submit any markdown plan file by path

Removes all configured plan-file state (module var, --plan-file flag,
/plannotator-set-file command, interactive path prompt). The agent now
writes its plan as a markdown file anywhere inside the working directory
and calls plannotator_submit_plan with the file path.

During planning, the write gate allows any .md / .mdx file inside cwd
(with traversal/absolute-escape rejection) instead of a single configured
path. plannotator_submit_plan gains a required filePath parameter, which
is validated, stat-checked, and read. Version history in ~/.plannotator
continues to key off plan content so naming is free-form without losing
version linking.

For provenance purposes, this commit was AI assisted.

Author: backnotprop