fix(prd): stop body lines being read as a PRD's title and status

[clawedassistant26]

A PRD whose body has a line starting with title: or status: gets those values shown in prd list and the README index, in place of its real front matter.

Repro

prd/0001-job-api.md — an ordinary PRD documenting an API, with a YAML sample in a fenced block:

---
title: "Job API v2"
status: Accepted
---

## Requirements

- R1 [P0] Return the job record:

```yaml
title: nightly-import
status: queued

`listPrds()` returns `title: "nightly-import", status: "queued"`. The README index table and the `prd list` output (both `bin/moshcode.mjs` and the TUI) show those instead of `Job API v2` / `Accepted`.

## Cause

`src/prd.mjs:214` read a fixed window of the first 16 lines and kept the **last** `^title:`/`^status:` match anywhere in it, never stopping at the closing `---`. Generated front matter is exactly 16 lines, so freshly created PRDs hide the bug; it appears as soon as the front matter is shorter, which any hand-written or trimmed PRD is.

## Fix

Parse the delimited front-matter block (leading `---` to its closing `---`) and scan only that. Two follow-on effects, both intended:

- A file with no front matter no longer takes a title or status out of prose; it falls back to the slug and `?`, which is what an undeclared status means.
- Front matter longer than 16 lines is now read in full.

Existing PRDs render byte-identically — verified against this repo's own `prd/` (0001-0004 unchanged).

## Tests

Two added to `test/prd.test.mjs`: the fenced-YAML case above, and the no-front-matter fallback. Stash-verified with `git stash push -- src/`: both **fail** unpatched, both pass with the fix. Full suite `node --test`: **151 pass / 0 fail** (main is 149).