refactor(docs): split requirements.md into per-section files under docs/memory/requirements/

[vybe]

Summary

`docs/memory/requirements.md` has grown to 2400+ lines covering 36 sections. As a single file it is becoming difficult to navigate, causes merge conflicts when multiple PRs add requirements simultaneously (e.g. #930 and #933 both added §35 in the same session), and makes targeted updates harder to review. Splitting it into per-section files under a `docs/memory/requirements/` directory eliminates the structural conflict surface and gives each capability area a clear owner file.

Context

The conflict on `requirements.md` that surfaced during the #930 / #933 review cycle (both PRs added §35) is a symptom of the file having outgrown the single-file model. The fix is structural: move to a directory where each requirement section lives in its own Markdown file named by its number and slug (e.g. `requirements/35-schedule-timeout-validation.md`), with a lightweight `requirements/README.md` index replacing the current top-of-file table of contents.

Acceptance Criteria

• `docs/memory/requirements/` directory created; each existing section (§1–§36+) moved to its own file named `-.md`
• `docs/memory/requirements/README.md` index lists all sections with one-line descriptions and links — replaces the top-level TOC in the old file
• `docs/memory/requirements.md` either removed or replaced with a redirect stub pointing at the new directory
• All cross-references to `requirements.md` inside the repo updated to point at the new paths (CLAUDE.md, architecture.md, feature-flows.md, any skill/agent prompts)
• Development workflow skills updated: any instruction that says "update `docs/memory/requirements.md`" is changed to "add or update the relevant file under `docs/memory/requirements/`"
• `.claude/` submodule skills that reference requirements location are updated (e.g. `validate-pr`, `implement`, `autoplan`, `cso` prompts)
• New section convention documented: when a PR adds a new requirement, it creates a new numbered file rather than appending to a monolithic file; numbering convention defined (sequential integers, no gaps)

Technical Notes

• The `docs/memory/requirements/` path should be added to the `State Dependencies` tables in any skill that reads requirements (validate-pr, implement, autoplan, sprint, cso)
• Skill prompts that currently read `docs/memory/requirements.md` should be updated to read either the index or the specific section file relevant to the PR under review
• Consider whether the index `README.md` should be auto-generated from the section files or hand-maintained; hand-maintained is simpler and consistent with the existing docs model
• Migration can be done in a single PR since it is a pure rename/split — no content changes, just structure

[vybe]

Grooming: superseded — the requirements.md split this issue asks for shipped as #1406. docs/memory/requirements/ now holds per-area files with requirements.md as the index, exactly the structure proposed here.