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/shared/schema.sql

@@ -0,0 +1,57 @@
+-- z3agent schema v1
+
+PRAGMA journal_mode=WAL;
+PRAGMA foreign_keys=ON;
+
+CREATE TABLE IF NOT EXISTS runs (
+    run_id      INTEGER PRIMARY KEY AUTOINCREMENT,
+    skill       TEXT NOT NULL,
+    input_hash  TEXT,
+    status      TEXT NOT NULL DEFAULT 'running',
+    duration_ms INTEGER,
+    exit_code   INTEGER,
+    timestamp   TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_runs_skill ON runs(skill);
+CREATE INDEX IF NOT EXISTS idx_runs_status ON runs(status);
+
+CREATE TABLE IF NOT EXISTS formulas (
+    formula_id  INTEGER PRIMARY KEY AUTOINCREMENT,
+    run_id      INTEGER REFERENCES runs(run_id) ON DELETE CASCADE,
+    smtlib2     TEXT NOT NULL,
+    result      TEXT,
+    model       TEXT,
+    stats       TEXT,
+    timestamp   TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_formulas_run ON formulas(run_id);
+CREATE INDEX IF NOT EXISTS idx_formulas_result ON formulas(result);
+
+CREATE TABLE IF NOT EXISTS findings (
+    finding_id  INTEGER PRIMARY KEY AUTOINCREMENT,
+    run_id      INTEGER REFERENCES runs(run_id) ON DELETE CASCADE,
+    category    TEXT NOT NULL,
+    severity    TEXT,
+    file        TEXT,
+    line        INTEGER,
+    message     TEXT NOT NULL,
+    details     TEXT,
+    timestamp   TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_findings_run ON findings(run_id);
+CREATE INDEX IF NOT EXISTS idx_findings_category ON findings(category);
+CREATE INDEX IF NOT EXISTS idx_findings_severity ON findings(severity);
+
+CREATE TABLE IF NOT EXISTS interaction_log (
+    log_id      INTEGER PRIMARY KEY AUTOINCREMENT,
+    run_id      INTEGER REFERENCES runs(run_id) ON DELETE SET NULL,
+    level       TEXT NOT NULL DEFAULT 'info',
+    message     TEXT NOT NULL,
+    timestamp   TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_log_run ON interaction_log(run_id);
+CREATE INDEX IF NOT EXISTS idx_log_level ON interaction_log(level);