feat: add Guard Status Tracker workflow for issue #1711 (#1938)

Adds a daily agentic workflow that keeps the [Guards and Integrity
tracking issue
(#1711)](#1711) up to date
with the current codebase state.

## What it does

Runs weekdays at 8 AM UTC (also manually triggerable). Each run:

1. **Inventories source code** — counts files, lines, and tests across
all 4 guard components (DIFC engine, guard framework, guard config, Rust
GitHub Guard)
2. **Scans recent PRs** — finds guard-related merged/open PRs from the
last 14 days
3. **Compares with previous run** — uses cache-memory to detect deltas
(new PRs, LOC changes)
4. **Posts a status comment** on issue #1711 with a table + delta
summary

## Status comment format

| Component | Files | Lines | Tests |
|-----------|-------|-------|-------|
| DIFC Engine | X | Y | Z |
| Guard Framework | X | Y | Z |
| ... | ... | ... | ... |

Plus: recent PR list, architecture summary, and open items check.

## Configuration
- `strict: true`, scoped to `github/gh-aw-mcpg` with `min-integrity:
unapproved`
- `add-comment` safe output (max 1 per run) targeting issue #1711
- `cache-memory` for state tracking between runs

Author: lpcox