Add a Yaak CLI skill for coding agents, installed by yaak agent install

Teaches agents to drive the CLI: workspaces, environments, requests,
sending, response chaining, and importing. SKILL.md stays lean with
four references loaded on demand.

The skill is embedded in the binary and written to ~/.agents/skills plus
any detected tool directory, so it ships with the CLI and refreshes on
update. Reinstalls keep files edited locally unless --force.

Also fixes the `request schema http` hint for URL path parameters, which
said to omit the leading colon. Names without the colon are sent as query
string parameters instead, leaving the placeholder literal in the path.

crates-cli/yaak-cli/src/commands/mod.rs

@@ -1,3 +1,4 @@
+pub mod agent;
 pub mod auth;
 pub mod cookie_jar;
 pub mod environment;