[Deps] Put clap behind an optional cli feature

[kageiit]

clap sits in [dependencies] but only bin/server.rs and bin/client.rs use it - nothing under src/ mentions it. Every library consumer of mcp_rs has therefore been compiling clap and its subtree for nothing. This makes it optional behind a non-default cli feature and gates the two binaries on it.

required-features means cargo skips a binary rather than failing when the feature is off, so cargo build on a checkout still succeeds, and both CI jobs already pass --all-features, so the binaries stay built and linted.

Measured on this branch: cargo tree -e normal goes from 13 clap-subtree crates to zero, and cargo tree -e normal --features cli still shows them.

In gitar this subtree reaches most of the workspace, since mcp_rs is a dependency of agent-models. We just moved every gitar CLI from clap to usage, and this was the one remaining edge keeping clap in the graph.

Heads up on two things I did not touch:

CI is already red on main. cargo test --all-features fails with 15 E0061s in tests/tools.rs, from agent_id: Uuid landing on CallToolArgs as a required TypedBuilder field without the test call sites being updated. I reproduced that on main at 9b4756f with this branch stashed, so it predates this PR.

Left alone, as the fix is a semantic call: either the tests supply an agent_id or the field takes a #[builder(default)], and whoever wrote it knows which.

gitar is pinned to a commit that is not on any branch. Our rev = "43eb26e6" is the pre-squash head of #19, reachable only through refs/pull/19/head. #19 squash-merged as b496f16, and main has three commits since.

So consuming this means moving off that dangling head onto main, which also brings e6409b7 (sync get_tool) and 9b4756f. Separate bump on our side, not something this PR does.