fix(cli): exit via process::exit to avoid tokio stdin-blocking-pool hang

[bug-ops]

Summary

• mcpls did not exit on SIGTERM/SIGINT while a stdio MCP client kept its stdin write end open. rmcp::transport::stdio() is backed by tokio::io::stdin(), which internally parks an uncancellable spawn_blocking thread in a raw read() syscall; #[tokio::main]'s generated wrapper blocks in Runtime::drop waiting for that thread once main's body returns, even though all real shutdown work (LSP server teardown, log flush) had already completed. main now calls std::process::exit as its final step instead of returning normally. Scoped to crates/mcpls-cli/src/main.rs only; mcpls-core's serve/serve_with/shutdown/run_stdio are unaffected.
• Also fixes await_lsp_init_handle's timeout branch, which called JoinHandle::abort() without a subsequent await: a SIGTERM arriving mid-spawn_batch could let process::exit run before the runtime dropped the aborted task's locals (including not-yet-registered LSP Child handles relying on kill_on_drop), orphaning those processes — the exact failure mode fix(transport): handle SIGINT/SIGTERM in stdio transport and gracefully shut down LSP servers #270 guards against. abort() is now followed by a bounded re-await.
• Updated public doc examples (lib.rs, transport.rs, crates/mcpls-core/README.md) that previously taught the pre-fix hanging pattern.
• Added an e2e regression test (test_e2e_sigterm_exits_promptly_while_client_stdin_open, #[cfg(unix)]) that spawns the real binary, completes an MCP handshake, sends SIGTERM, and asserts prompt exit with status 0.

Fixes #308. Filed #318 as a separate, non-blocking follow-up for a distinct pre-existing race surfaced during test hardening (SIGTERM arriving before run_stdio's signal handler registers gets OS default disposition).

Test plan

• cargo +nightly fmt --all -- --check
• cargo clippy --all-targets --all-features --workspace -- -D warnings
• cargo nextest run --workspace --all-features --lib --bins (639/639)
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --all-features
• New e2e regression test run explicitly (--run-ignored ignored-only), 8/8 consecutive passes
• Adversarial critique (rust-critic) and code review (rust-code-reviewer) both completed, all findings addressed