Make AgentRunner the only Agent execution path

[gold-silver-copper]

Summary

• make AgentRunner the only execution path for configured Agent instances
• remove the raw Completion and StreamingCompletion traits and their Agent implementations
• keep raw, hook-free provider transport available explicitly through CompletionModel
• make execution-critical Agent state private and add runner-backed per-request overrides
• route Extractor through the full hook lifecycle while preserving retry, usage, schema, and provider request behavior
• migrate manual AgentRun examples/tests to explicitly raw sans-I/O provider workflows

Why

Agent::completion() returned a mutable provider request builder without running the AgentRunner lifecycle. That allowed configured agent hooks, tool policies, termination, scratchpad state, and response/streaming events to be bypassed. A configured Agent should have one lowest-level execution path with one lifecycle contract.

Manual AgentRun examples remain intentionally provider-level sans-I/O workflows; this PR does not claim runner-backed durable stepping/checkpointing.

User impact

This is a breaking API change. Callers executing an Agent should use AgentRunner, Prompt, Chat, typed prompting, or streaming prompting. Callers that intentionally need raw provider transport should retain and call a CompletionModel directly.

Runner/request wrappers now expose history, temperature, max-token, additional-parameter, and tool-choice overrides without permitting post-hook request mutation.

Review findings addressed

An independent full-diff review found and this branch fixes:

• StreamingPromptRequest::new discarded the Agent's configured hooks; it now preserves them, with a regression proving a terminating Agent hook prevents provider I/O.
• non-object per-run additional_params overrides could be ignored; object values now merge and other values replace the baseline, with regression coverage.

The final independent review found no remaining P0/P1 issues or confirmed hook-parity, extractor/provider-compatibility, security, or raw-Agent bypass defects.

Validation

• cargo fmt --all
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• cargo test -p rig-core
• cargo test -p rig --all-features --test gemini gemini::cassette::extractor -- --nocapture --test-threads=1
• cargo test -p rig --all-features --test gemini gemini::cassette::agent_run -- --nocapture --test-threads=1
• cargo doc --workspace --no-deps
• git diff --check

Credential-dependent live tests remain ignored as designed. Workspace docs retain one unrelated pre-existing rig-neo4j private intra-doc-link warning.