Handle the Grok agent

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Why

agent-abstraction gained Agent::Grok and this crate's matches did not, so building against 0.4.21 fails outright:

error[E0004]: non-exhaustive patterns: `Agent::Grok` not covered
error: could not compile `agency-proxy` (lib)

This is currently taking AgencyZero's CI down — both Rust and Panel checks fail. scripts/stage-agency-proxy-sidecar.sh builds this crate from crates.io at the version agency-proxy-client pins, and cargo resolves it against the newer abstraction, so the published 0.1.9 cannot compile.

What

• agent_name and agent_for_provider gain the Grok arm.
• The two registry sweeps that enumerate agents (account_usage, probe_providers) include it, so a Grok run reports account usage and probe status like the other three.
• agent-abstraction was a path dependency into a sibling checkout, which no CI can resolve. It takes the published 0.4.21, which is where Agent::Grok is.
• Workspace version to 0.1.10, since 0.1.9 is published and AgencyZero builds this from crates.io.

Verified

• cargo check -p agency-proxy against published agent-abstraction 0.4.21
• cargo test --workspace — 24 passing
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo fmt --check — clean

AgencyZero needs 0.1.10 published before its CI goes green.

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Folded into #9 so this repo has one PR. The Grok fix is cherry-picked onto that branch, verified there: builds at 0.1.10 against published agent-abstraction 0.4.21, 24 tests, clippy -D warnings clean, fmt clean, and --locked passes with the lockfile committed.