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/init.rs

@@ -137,6 +137,7 @@ fn scaffold_manifest(slug: &str, name: &str) -> Manifest {
         }],
         triggers: crate::manifest::ManifestTriggers::default(),
         secrets: crate::manifest::ManifestSecrets::default(),
+        vars: std::collections::BTreeMap::new(),
     }
 }