Document local observability model

[codeforester]

Summary

• Add docs/observability.md to define Base's future local command history, last-error explanation, and diagnostic report model.
• Clarify that observability data stays local by default, keeps basectl logs as raw evidence, and uses redacted metadata/reporting for future surfaces.
• Link the design from the docs map, README logs section, and changelog.

Validation

• env -u BASE_HOME ./bin/base-test baseline before docs edits: Python 366 passed, 1 skipped; BATS 393 passed
• git diff --check
• git diff --cached --check

Demo Impact

• None. Documentation-only design work.

Closes #396