refactor(client)!: single canonical CompletionClient + AgentClientExt

[gold-silver-copper]

TL;DR (net change): Collapse the duplicate rig-agent CompletionClient
into a supertrait-based AgentClientExt extension trait, leaving rig-core's
CompletionClient as the single canonical trait. rig::client::CompletionClient
still resolves and use rig::prelude::*; keeps working unchanged; no source
changes are required for facade users. No behavior change, no shadow, no
hand-maintained forward.

---

Follow-up to #2197. That PR, to preserve the pre-split client.agent(m)
ergonomics, introduced a second, same-named CompletionClient trait in
rig-agent that re-declared completion_model, hand-forwarded to the canonical
rig-core trait (with a "add a matching forward if core grows" maintainer
comment), and shadowed the core trait in the facade and preludes. This
removes that duplication while keeping the ergonomics.

What changed

• One canonical CompletionClient (rig-core). The classic agent() /
  extractor() constructors now live on a new AgentClientExt extension trait
  whose supertrait is rig_core::client::completion::CompletionClient — so it
  builds on completion_model / CompletionModel via the supertrait bound and
  there is nothing to keep in sync if the portable trait grows a method. The
  hand-written forward and its maintainer comment are gone.
• No trait shadowing. AgentClientExt and CompletionClient share no method
  names, so both resolve without ambiguity. The deliberate same-name shadow in
  the facade client module and both preludes is removed.
• rig::client re-exports the full rig-core client surface, including the
  canonical CompletionClient (pub use rig_core::client::*;). AgentClientExt
  is a distinct name layered on top, so this is one canonical trait with no
  shadow — not a re-introduced duplicate.
• Call sites standardized on the prelude. All sites that imported the client
  trait by bare path now use use rig::prelude::*; (or rig_agent::prelude::*
  for crates depending on rig-agent directly).

Migration

Facade users (most): no change required. use rig::prelude::*; gives the
full completion_model + agent + extractor surface, and
rig::client::CompletionClient still resolves. If you imported the client trait
explicitly, use rig::client::{CompletionClient, AgentClientExt} —
CompletionClient provides completion_model, AgentClientExt provides
agent / extractor. AgentModelExt::into_agent_builder is unchanged.

Provider clients no longer carry inherent .agent() / .extractor() methods;
they come from AgentClientExt (a blanket impl over every completion client),
which the prelude brings into scope.

Verification

• cargo check --workspace --all-features --all-targets — clean (0 warnings)
• cargo clippy --workspace --all-features --all-targets -- -D warnings — exit 0
  (no ambiguous-glob-reexport)
• cargo test --doc -p rig -p rig-agent -p rig-core --all-features — pass
• cargo test -p rig-agent --features test-utils, cargo test -p rig-runtime-conformance — pass
• cargo check -p rig --no-default-features --features {agent,derive,test-utils} — pass
• cargo check -p rig{-agent,} --target wasm32-unknown-unknown — pass
• tests/tool_facade_traits.rs covers both the single use rig::prelude::*;
  surface and the explicit rig::client::{CompletionClient, AgentClientExt} pair,
  each exercising completion_model + agent + extractor.
• A pure-rig consumer (no rig-core dep) compiles the README, prelude, and
  explicit import forms.