fix(pi): let agent submit any markdown plan file by path (#595)

* 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.

* refactor(pi): drop isSubmitPathAllowed alias

isSubmitPathAllowed was a pure pass-through to isPlanWritePathAllowed
with no plausible reason to diverge. Use the write-gate helper directly
from submit_plan and drop the alias.

For provenance purposes, this commit was AI assisted.

* fix(pi): update bundled planning prompt for filePath-based submit

The shipped plannotator.json planning prompt still referenced
${planFilePath} and instructed the agent to call plannotator_submit_plan
with no arguments. After the submit tool gained a required filePath
parameter, default sessions hit the stale prompt and submits failed.
Rewritten to match the new flow: agent picks its own plan filename and
passes it to plannotator_submit_plan.

For provenance purposes, this commit was AI assisted.

* fix(pi): persist state on plan denial

Mirror the approve branches by calling persistState() before returning
on the deny path. Without this, a Pi restart between denial and the
agent's next submit would lose the lastSubmittedPath in memory, forcing
the agent to re-pick a plan file instead of resuming on the same one.

For provenance purposes, this commit was AI assisted.

Author: backnotprop