feat(api): wire LlmClient injection seam; stop stderr writes; prune dead Planner trait

[ZhiXiao-Lin]

Framework-vs-host boundary tightening from an architecture audit of core. Independent of #67 (prompt hardening) — touches a disjoint set of files.

The audit's guiding rule: the framework owns mechanism/protocol; the host owns presentation/input-syntax/business-policy. A built-in should be a replaceable extension point only when a concrete present consumer needs it (no future-proofing).

Added — SessionOptions::with_llm_client(Arc<dyn LlmClient>)

The one real missing seam. LlmClient is already a public trait and the whole engine runs on Arc<dyn LlmClient>, but no public path let a host inject one — every session resolved via a hardcoded provider-string factory, and the only injecting constructor (build_session) is #[cfg(test)]. This violated the repo's own "Typed Extension Options" rule (backends must be swappable as typed objects, not selected by a raw provider name) — and it was the only Action-layer backend that did, while workspace/memory/store/security are all object-injectable.

• resolve_session_llm_client now returns a host-supplied client when present, else the existing provider/model factory (unchanged default).
• Unblocks: providers the factory doesn't cover, a deterministic record/replay client (mirroring FixedClock/SequentialIdGenerator), and proxy/audit HTTP wrappers — without forking core.

Fixed — framework no longer writes to the host's stderr

Replaced a stray eprintln!("[DEBUG] HTTP error...") in the OpenAI client (fired on every transport error, dumping [DEBUG] to every consumer's terminal) with tracing::error!, consistent with the rest of the crate.

Removed — dead Planner trait (planning::Planner)

Re-exported but had zero dyn Planner dispatch and no consumer; every call site uses LlmPlanner's inherent methods directly. Pruned per the pruning rule — the only real variability (the LLM) is now swappable via with_llm_client. LlmPlanner itself is unchanged.

Testing

• 2 new resolver unit tests: a host-supplied client bypasses provider/model config; the control case (no client + no default_model) errors.
• cargo test -p a3s-code-core --lib: 1757 passed, 0 failed. clippy clean. cargo fmt --check clean.

Notes

No version bump (left to the maintainer's release flow). Slash-command de-UX (the larger boundary item from the same audit) is intentionally not included — it changes a public behavior surface and needs a product decision first.