Create Base product requirements document

[codeforester]

Summary

• Add docs/product-requirements.md as the maintained PRD for accepted Base product intent, target users, core jobs, durable requirements, non-goals, success criteria, and PRD maintenance rules.
• Link the PRD from README.md, docs/README.md, and .ai-context/INDEX.md.
• Keep implementation detail in existing technical docs by linking outward instead of duplicating architecture content.

Issue

Closes #1004

Validation

• git diff --check
• git diff --check --cached

Demo Impact

None. Documentation-only change.

Notes

Opened as a draft PR so the PRD remains blocked on maintainer review before merge. .ai-context/ is updated because this adds a canonical product-shape document.

Checklist

• Branch name follows <category>/<issue>-<YYYYMMDD>-<slug>.
• PR is scoped to one issue, unless a documented multi-issue exception applies.
• PR body explains what changed and how it was validated.
• Validation commands were run from the current checkout or worktree, or unavailable checks are explained.
• Relevant BATS and Python tests pass. Documentation-only change; not applicable.
• Bug fixes include regression proof or a documented reproduction when practical. Not applicable.
• Documentation is updated when behavior or user-facing commands change.
• AI context is updated in .ai-context/, or the PR body explains why it is not applicable.
• PR includes Fixes #<issue> or Closes #<issue> when it should close the issue.
• Demo Impact is meaningful for needs-demo work, or explicitly says None.