fix(agent): show progress for remote sessions and say bindings in local agent runs

[radim10]

Summary

• agent run --remote showed nothing while the Agent Proxy session was created. The request runs after the startup warnings, so a slow or failing one left the terminal silent until the error appeared. It now shows Starting remote agent session... during the request and ✓ Remote agent session started (N bindings) once the session opens. The spinner is cleared before any error prints, so text and --json errors look the same as before.
• A local agent run now says "bindings", not "environment". It loads only its profile's bindings, so the spinner reads Loading bindings... and the completion line ✓ Bindings loaded (N bindings). Plain run keeps Loading environment... / Environment loaded (N secrets), and egress-only profiles are unchanged.
• The remote wording says "starting", not "loading", on purpose. Remote binding values never reach the machine; the control plane holds them, so "loading" would be misleading there.
• Moved two doc comments in src/handlers/run/entry.rs back above the functions they describe.

Test plan

• Unit tests for the remote started message and for the local loading/loaded messages (singular and plural)
• cargo fmt --check passes
• cargo test passed in full before the remote success line was added. After it, the new test passes; the rest of the suite was only run inside a sandbox that blocks local port binding, where the only failures are the proxy, subprocess, telemetry and session tests that need it
• Checked by hand: agent run --remote shows the spinner and then the started line