Fix fix-plan cleanup script bugs around duplicate section headings

[DrumRobot]

Problem

A tracking file processed by the fix-plan cleanup.py script can end up with the same top-level section heading (e.g. a recurring-tasks section) appearing twice, usually from independent sessions each appending content without checking whether the heading already exists. Two concrete bugs were found once that state occurred:

1. Duplicate-heading insertion bug: cleanup.py's completed-item archival step inserts a fresh "Completed" section heading before every occurrence of the recurring-tasks heading it finds, with no guard checking whether a "Completed" section was already inserted earlier in the same run. When the recurring-tasks heading appears twice, running the script produces a duplicated "Completed" section instead of a single consolidated one.
2. False-positive schema warning: A separate hook that checks tracking-file items for a required Why/How-to-apply structure fires an advisory warning on recurring-task items too, even though recurring-task items intentionally follow a different schema (condition/procedure/cost), not the Why/How-to-apply schema the check expects.

Scope

1. Fix the archival step in cleanup.py to track whether it has already inserted a "Completed" section during the current run (e.g. an inserted_completed flag) and skip re-inserting it on a second occurrence of the recurring-tasks heading.
2. Add an exception to the schema-check hook so recurring-task items (identified by their own marker/tag convention) are validated against their own condition/procedure/cost schema instead of the Why/How-to-apply schema, avoiding the false-positive advisory.
3. Add a regression test/fixture for each fix: (a) a fixture tracking file with two occurrences of the same recurring-tasks heading, verifying only one "Completed" section is produced after archival; (b) a fixture recurring-task item, verifying the schema-check hook does not fire its advisory on it.

Verification

Feature	Procedure	Expected Result
No duplicate Completed section	Run the archival step against a fixture file with a duplicated recurring-tasks heading	Exactly one "Completed" section exists in the output, not one per heading occurrence
Schema check no longer false-positives on recurring-task items	Run the schema-check hook against a fixture recurring-task item (condition/procedure/cost schema, no Why/How-to-apply fields)	No advisory warning fires
Existing schema-check behavior preserved	Run the schema-check hook against a fixture normal item missing Why/How-to-apply	Advisory warning still fires as before
Existing cleanup fixtures	Run the cleanup script's existing test suite	All existing tests still pass