Add Copilot skill architecture with 10 skills, 2 agents, and shared infra

Introduce .github/skills/ with solve, prove, optimize, simplify,
encode, explain, benchmark, memory-safety, static-analysis, and
deeptest skills. Each skill follows a SKILL.md + scripts/ pattern
with Python scripts backed by a shared SQLite logging library
(z3db.py). Two orchestrator agents (z3-solver, z3-verifier) route
requests to the appropriate skills.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/skills/explain/SKILL.md

@@ -0,0 +1,52 @@
+---
+name: explain
+description: Parse and interpret Z3 output for human consumption. Handles models, unsat cores, proofs, statistics, and error messages. Translates solver internals into plain-language explanations.
+---
+
+Given raw Z3 output (from the **solve**, **prove**, **optimize**, or **benchmark** skills), produce a structured explanation. This skill is for cases where the solver output is large, nested, or otherwise difficult to read directly.
+
+# Step 1: Identify the output type
+
+| Output contains | Explanation type |
+|----------------|-----------------|
+| `(define-fun ...)` blocks | model explanation |
+| unsat core labels | conflict explanation |
+| `:key value` statistics | performance breakdown |
+| `(error ...)` | error diagnosis |
+| proof terms | proof sketch |
+
+# Step 2: Run the explainer
+
+```bash
+python3 scripts/explain.py --file output.txt
+python3 scripts/explain.py --stdin < output.txt
+python3 scripts/explain.py --file output.txt --debug
+```
+
+The script auto-detects the output type and produces a structured summary.
+
+# Step 3: Interpret the explanation
+
+For models:
+- Each variable is listed with its value and sort
+- Array and function interpretations are expanded
+- Bitvector values are shown in decimal and hex
+
+For unsat cores:
+- The conflicting named assertions are listed
+- A minimal conflict set is highlighted
+
+For statistics:
+- Time breakdown by phase (preprocessing, solving, model construction)
+- Theory solver load distribution
+- Memory high-water mark
+
+# Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| file | path | no | | file containing Z3 output |
+| stdin | flag | no | off | read from stdin |
+| type | string | no | auto | force output type: model, core, stats, error |
+| debug | flag | no | off | verbose tracing |
+| db | path | no | .z3-agent/z3agent.db | logging database |