docs: add DataWorkingLayer rule and fix UseForge skill-asset guidance

[N4M3Z]

Problem

UseForge still says assembly only deploys .md files and tells you to put skill helpers in bin/. ASSEMBLY-0012 in forge-cli changed that: a skill directory deploys its whole tree, only the top-level SKILL.md is assembled, everything else is copied verbatim. The rule now points the wrong way, and a helper parked in bin/ to satisfy it misses the ${CLAUDE_SKILL_DIR} pattern that works now.

DataWorkingLayer never existed. MarkdownExtract (forge-text) and BuildDeck (forge-docs) both name the ~/Documents, ~/Data, vault convention, but no rule defined it, so the references dangled.

Fix

Add rules/DataWorkingLayer.md: originals stay in ~/Documents, the extracted working layer lives in ~/Data/<Type>/<project>/, and the vault references that layer instead of copying it.

Rewrite the UseForge assembly paragraph for verbatim deploy. Non-UTF-8 binaries are skipped with a warning, the executable bit isn't preserved yet so a deployed helper runs through its interpreter, and compiled binaries still belong in bin/.

Out of scope

My working copy also has a --force paragraph edit to UseForge from separate work. It's left out. This branch resets UseForge to main first, then changes only the assembly line.

Test plan

• Built forge-cli and ran forge install on a skill bundling a UTF-8 uv run helper. It deployed (as 644, so the exec-bit caveat holds).
• jj diff shows only the two rule files.
• CI: gitleaks, semgrep, forge validate.