fix(skills): embed skill content in the binary for single-binary installs

`skills get core` (the first step the agent-browser skill stub tells agents to
run) failed with "Skills directory not found" on a GitHub-Release / install.sh
install: only the binary is shipped, with no adjacent skills/ or skill-data/
the way an npm install bundles them, so find_package_root() returned nothing.

Embed skills/ and skill-data/ into the binary via include_dir (168K) and, when
no on-disk skill dirs are found, extract them once to a per-version cache dir
($CACHE/agent-browser/skills-<version>/) and serve from there. npm/dev installs
still use the on-disk dirs unchanged.

Verified: from an isolated dir (no skills/ nearby), `skills list` shows all 6
skills and `skills get core` serves content.

cli/Cargo.toml

@@ -19,6 +19,7 @@ serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 regex-lite = "0.1"
 dirs = "5.0"
+include_dir = "0.7"
 base64 = "0.22"
 getrandom = "0.2"
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "net", "io-util", "time", "sync", "signal", "process"] }