feat(cli): declare app vars in the manifest + plan/apply them

Adds a `[vars]` block to `picloud.toml` (key → TOML value), reconciled by
`pic plan`/`pic apply` alongside scripts/routes/triggers/secrets. Unlike
`[secrets]` (name-only), var values live inline since they aren't secret.
The overlay (`picloud.<env>.toml`) may carry `[vars]`; overlay keys win
per key (the env-specific value of an env-agnostic default).

`build_bundle` serializes the vars to JSON for the wire (TOML round-trips
via serde); `pic plan` shows a `var` row group and `pic apply` reports
`vars +c ~u -d`. The `PlanDto`/`ApplyReportDto` gain the matching fields.
`pic pull` carries an empty `[vars]` for now (export lands next). Adds a
round-trip + overlay-precedence unit test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/picloud-cli/src/cmds/plan.rs

@@ -95,11 +95,22 @@ pub fn build_bundle(manifest: &Manifest, base_dir: &Path) -> Result<Value> {
         triggers.push(tagged("queue", s)?);
     }
 
+    // Vars: key → JSON value. TOML values round-trip to JSON via serde so the
+    // wire shape matches the server's `serde_json::Value`.
+    let mut vars = Map::new();
+    for (k, v) in &manifest.vars {
+        vars.insert(
+            k.clone(),
+            serde_json::to_value(v).with_context(|| format!("encoding var `{k}`"))?,
+        );
+    }
+
     Ok(json!({
         "scripts": scripts,
         "routes": routes,
         "triggers": triggers,
         "secrets": manifest.secrets.names,
+        "vars": vars,
     }))
 }
 
@@ -114,11 +125,12 @@ fn tagged(kind: &str, spec: impl Serialize) -> Result<Value> {
 
 fn render(plan: &PlanDto, mode: OutputMode) {
     let mut table = Table::new(["kind", "op", "resource", "detail"]);
-    let groups: [(&str, &Vec<ChangeDto>); 4] = [
+    let groups: [(&str, &Vec<ChangeDto>); 5] = [
         ("script", &plan.scripts),
         ("route", &plan.routes),
         ("trigger", &plan.triggers),
         ("secret", &plan.secrets),
+        ("var", &plan.vars),
     ];
     for (kind, changes) in groups {
         for c in changes {