docs: focus each file on users or coworkers, drop unrelated content (#18)

project-roadmap.md went from a 146-line phase-by-phase history to a
48-line focused view: what's planned, what's out of scope. Completed
work is already in git log + GitHub Releases — the doc re-telling it
was the thing most likely to rot and least likely to be read.

project-overview-pdr.md: "Open questions" section dropped its stale
bullet list and now just points at project-roadmap.md (single source of
truth for planned work).

code-standards.md: drop the ".claude/ directory" commit rule — that's
a per-user workflow detail, not a project-level standard. Docs are for
users of the CLI/Action and coworkers of this repo, nothing else.

docs/code-standards.md

@@ -73,7 +73,6 @@ Never write comments that describe what well-named code does (`// increment coun
 
 - Conventional commit prefixes: `feat:`, `fix:`, `refactor:`, `docs:`, `test:`, `build:`, `ci:`, `chore:`.
 - Scope is optional: `refactor(card): …`.
-- Never use `chore:` or `docs:` for `.claude/` directory changes (project rule).
 - No AI references. No "generated by" footers. Keep the subject ≤ 72 chars.
 
 ## Pre-commit checklist