docs(plan): implementation plan for the wait-for readiness gate

Six TDD tasks: schema + KDL parsing, the Waiting state and WaitInfo, a
ready module that evaluates one condition, the interruptible gate inside
do_start, CLI surfacing, and the README.

Also corrects the spec: xy-protocol has no etcetera dependency, so ~ is
expanded from $HOME; and in KDL `command`/`args` are sibling nodes inside
the wait-for block, which changes how the parser counts conditions.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EGntTHCW3sEPy1VBRopNNp

docs/superpowers/specs/2026-08-07-xy-readiness-gate-design.md

@@ -66,10 +66,25 @@ Exactly one condition per `wait-for` block. Zero or more than one is a parse
 error, so `xy reload` exits 3 and changes nothing — matching how invalid
 config already behaves.
 
-`path` values get `~` expanded at parse time via `etcetera::home_dir()`, since
-the motivating case is literally `~/.orbstack/run/docker.sock`. The expanded
-path is what `xy status` displays, so there is no ambiguity about what was
-checked.
+`path` values get a leading `~/` expanded at parse time from `$HOME`, since the
+motivating case is literally `~/.orbstack/run/docker.sock`. `$HOME` rather than
+`etcetera::home_dir()` deliberately: `xy-protocol` is a leaf crate with no
+directory dependencies, and adding one for a two-line prefix substitution is
+not worth it. If `$HOME` is unset the literal path is kept, which simply never
+exists and surfaces as a normal timeout. The expanded path is what `xy status`
+displays, so there is no ambiguity about what was checked.
+
+Note the KDL shape of the `command` kind: `command` and `args` are **sibling
+nodes** inside the block, mirroring the top level, not one node with trailing
+arguments:
+
+    wait-for {
+        command "docker"
+        args "info"
+    }
+
+so the parser accumulates them separately and combines them after the loop.
+`args` without `command` is a parse error.
 
 The `command` kind is bounded by the poll `interval`: each invocation is run
 under a timeout of `interval` and a non-exit is treated as not-ready, so a hung