docs: finish private working-note archive and retain active guidance

[witbrock]

Outcome

Finish the retained public/private documentation change from d0ecd3f8 and f032ec4e. Remove 68 tracked working notes already preserved in the private research archive, keep public implementation guidance self-contained, repair historical citations and restore the optional ignored AGENTS_Private.md discovery hook.

The old branch also deleted the Jira-to-Von migration guide. This reconciliation preserves its current contents byte-for-byte and keeps newer public index routes, including task retrieval, coding-agent assignment, rumination and Mobile Web Push. The private archive contains 71 entries: the 68 removed public files, two originally untracked notes and the historical copy of the migration guide. No private text is added to this PR.

Validation and decision

Tier 0. Merge decision changes — the stale deletion/link risks are resolved and proportionate evidence supports publication:

• All 71 archived destination hashes verified against the private repository's origin/main; its original archive commit is already an ancestor.
• All 68 removed tracked files were unchanged since the archived public baseline.
• 22 commit-pinned historical Markdown references resolve to Git objects; no remaining Markdown links target the removed files.
• Current migration guide is byte-identical to the baseline.
• Private guide is ignored/untracked; Python changes are only two historical-reference docstrings, with executable AST unchanged.
• git diff --check passes.

Required CI remains the final publication check. No runtime behaviour or deployment claim. Historical public Git content remains public history; relocation does not erase it.