Standardize Core Document maintenance headers and index output formats

[codeforester]

Summary

• Apply Status: and Last reviewed: metadata to the eight undated Core Documents.
• Refresh the Tool Boundaries evidence date.
• Add output-formats.md to the complete Core Documents index.
• Keep this PR stacked on the positioning migration so the shared docs index remains conflict-free.

Issue

Fixes #1760

Validation

• git diff --check
• /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest -q tests/test_docs_version.py tests/test_base_cli_docs.py — 2 passed
• Verified all eight Core Documents have maintenance headers.
• Verified output-formats.md appears exactly once in docs/README.md.

Demo Impact

None. This is documentation metadata and indexing only.

Notes

This draft targets documentation/1757-20260725-positioning-migration as the preceding train branch. Retarget it to main after #1757 is merged.