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.lock

@@ -57,6 +57,7 @@ dependencies = [
  "hex",
  "hmac",
  "image",
+ "include_dir",
  "libc",
  "regex-lite",
  "reqwest",
@@ -1048,6 +1049,25 @@ version = "1.12.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e7c5cedc30da3a610cac6b4ba17597bdf7152cf974e8aab3afb3d54455e371c8"
 
+[[package]]
+name = "include_dir"
+version = "0.7.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "923d117408f1e49d914f1a379a309cffe4f18c05cf4e3d12e613a15fc81bd0dd"
+dependencies = [
+ "include_dir_macros",
+]
+
+[[package]]
+name = "include_dir_macros"
+version = "0.7.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7cab85a7ed0bd5f0e76d93846e0147172bed2e2d3f859bcc33a8d9699cad1a75"
+dependencies = [
+ "proc-macro2",
+ "quote",
+]
+
 [[package]]
 name = "indexmap"
 version = "2.13.0"