release: 0.10.0 — deprecate old constructors, migrate to from_config

[yuanhao]

What

Completes Phase B. The config-first construction API (shipped additively in #59) becomes the recommended path: the old builders are now #[deprecated(since = "0.10.0")], and every call site across code and docs migrates to from_config.

Deprecated (removed in 1.0 — still work, with a compiler nudge)

• Agent::new, Agent::with_model, Agent::with_model_config
• SubAgentTool::new, SubAgentTool::with_model, SubAgentTool::with_model_config

The migration

// before (0.9): pair provider+config by hand, model id twice
let agent = Agent::new(OpenAiCompatProvider)
    .with_model_config(ModelConfig::zai("glm-4.7", "GLM 4.7"))
    .with_model("glm-4.7")
    .with_api_key(key);

// after (0.10): provider inferred from config.api; key from ZAI_API_KEY
let agent = Agent::from_config(ModelConfig::zai("glm-4.7", "GLM 4.7"));

Full before/after table in CHANGELOG.md. with_api_key is not deprecated.

Sweep (CI is -Dwarnings, so every compiled site moved here)

• Examples (code_review, rlm, shared_state) + both src doctests.
• Tests — all migrated except two kept under #[allow(deprecated)] to retain coverage of the deprecated-but-present API until 1.0: test_agent_builder_pattern (asserts the builder's effect) and session_cost_usd_none_without_model_config (the no-config None branch is only reachable via Agent::new).
• All 19 mdBook pages (providers/concepts/guides/reference). Azure/Bedrock/Vertex use ModelConfig::custom (no preset exists).
• SubAgentTool from_* route through a private build() so they don't trip their own deprecation lint.

Release bits

• Version 0.10.0; CHANGELOG.md with migration guide.
• README install 0.9 → 0.10; docs/installation 0.5 → 0.10 + MSRV 1.56 → 1.86; README OpenAI-compat example now builds via from_config.

Verification

• cargo clippy --all-targets --all-features (-Dwarnings) — clean
• cargo test --all-features — all green
• cargo doc (-Dwarnings) + doctests — clean

Not in this PR

The crates.io publish (tag + GitHub Release, which triggers publish.yml) is held for maintainer sign-off, per the release flow.

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