Add schema-first API docs drift detector (xtask check-docs)

Documentation.md's Lua API and IPC protocol sections are hand-written
prose and have drifted from the actual bread.* binding / IPC method
surface before. This adds a checked-in registry (api-schema.toml) of
every current Lua binding and IPC method, plus a new xtask crate
(`cargo run -p xtask -- check-docs`) that cross-checks it against
breadd/src/lua/mod.rs, breadd/src/ipc/mod.rs, and Documentation.md
itself, failing loudly on any mismatch. It's a drift detector, not a
doc generator — Documentation.md's prose is untouched except for one
genuine gap the bootstrap surfaced: bread.hyprland.eval had no mention
anywhere in the docs, now given a one-line example alongside its
siblings.

CONTRIBUTING.md documents the resulting contributor workflow.

xtask/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "xtask"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[[bin]]
+name = "xtask"
+path = "src/main.rs"
+
+[dependencies]
+anyhow.workspace = true
+serde.workspace = true
+toml = "0.8"