fix: skip archive directories in tasks.py recursive walk

[benslockedin]

Summary

tasks.py walks recursively to find tasks.json files but doesn't skip archive/reference directories. Legacy files in these paths with incompatible schemas cause _read_json() to sys.exit(1), killing the entire save operation.

Changes

• _all_task_files() — added _archive, _references, 01_Archive, raw to skip_dirs
• _read_json() — added strict=False mode that returns None + prints a warning instead of exiting. Default remains strict=True for direct operations (add, done, edit)
• _collect_all_tasks() + _find_task() — use strict=False and skip None results gracefully

Test plan

• Run tasks.py list on a walnut with legacy tasks.json in archive paths — should list tasks, print warning for skipped files
• Run tasks.py add on same walnut — should succeed
• Run tasks.py done targeting a task in _kernel/tasks.json — strict mode still exits on malformed kernel file
• Run tasks.py add on a walnut with no archive cruft — works exactly as before

Splits the minimal crash fix from #28. The v2→v3 migration subsystem remains in #28 for separate review.

Reviewed by: Patrick Brosnan (@patrickSupernormal) — original bug analysis in merge safety audit (fn-8-vly).