fix(prd): stop a --> in the idea closing the seed comment early

[clawedassistant26]

renderPrd embeds the idea verbatim in the hidden <!-- seed: ... --> marker (src/prd.mjs:113). If the idea itself contains -->, the comment closes at that terminator, so the remainder of the idea and the template's real closer land in the rendered output as visible text.

Repro (on main)

moshcode prd "ship a parser for --> arrows in specs"

writes into prd/0001-ship-a-parser-for-arrows-in-specs.md:

## Problem

<!-- seed: ship a parser for -->
 arrows in specs -->
_Describe the user/business problem, and why it matters now._

The arrows in specs --> line is outside the comment and renders in the Problem section of every new PRD created from such an idea. Any idea quoting an HTML comment, an arrow, or a spec snippet hits it.

Fix

A small commentSafe() helper escapes only the terminator (--> → -->) and only inside the seed marker, where it is never displayed. Ideas that contain no --> render byte-identically, so existing PRDs and prd/README.md are unaffected. Nothing reads the marker back, so no parsing changes.

Tests

Two regression tests in test/prd.test.mjs — one at renderPrd level, one end-to-end through createPrd — asserting the seed line holds exactly one terminator and that the line after it is still the untouched template placeholder.

• git stash push -- src/ (tests only, unpatched source): 2 fail, 4 pass
• With the fix: 6/6 pass
• Full suite node --test: 149 pass / 0 fail (was 147)