chore(types): mypy strict for 5 modules (batch 1 of #34)

[fede-kamel]

First batch of issue #34 — migrate mypy `ignore_errors` blanket to per-line.

Lifted modules (1066 LOC, all in strict mode now)

• `locus.core.reducers` (445 LOC)
• `locus.core.send` (322 LOC)
• `locus.models.registry` (150 LOC)
• `locus.tools.context` (67 LOC)
• `locus.tools.registry` (82 LOC)

Real fixes (not type-ignore covers)

• `core/reducers.py` — `AddMessages.call` now narrows the str sentinel up front, accepts `list[Message] | Message | str`, and raises on unrecognized strings (replaces the buggy `[update]` wrap that was producing `list[str]` for non-list inputs).
• `core/protocols.py` + `models/base.py` — `ModelProtocol.stream()` declared as `def` (not `async def`) so the Protocol matches concrete async-generator implementations. `async def` with `yield` returns `AsyncIterator[X]` directly, not `Coroutine[..., AsyncIterator[X]]`; the Protocol now types correctly.
• `tools/registry.py` — `iter` return type annotated as `Iterator[Tool]` (was missing).
• `tools/context.py` — `confidence` property cast to `float` at the boundary so the `Any`-typed `self.state` doesn't poison the return.

Targeted `# type: ignore[code]` annotations (with justification comments)

• `MaxValue` / `MinValue`: unbounded `T` can't prove `SupportsRichComparison`.
• `get_args(annotation)[1]`: `typing.get_args` returns `tuple[Any, ...]`.
• `is_send_list()`: custom TypeGuard-style narrowing mypy can't propagate.
• `ToolRegistry.iter`: intentional Liskov override of Pydantic's field iterator.
• `models/registry` lambdas + `_make_oci`: `cast("ModelProtocol", ...)` at the registration boundary because mypy's `Callable`-variance check doesn't propagate structural-Protocol narrowing on Pydantic model classes.

Test plan

• `hatch run typecheck` — "Success: no issues found in 150 source files"
• `hatch run test` — 3,246 unit tests pass + 1 skip
• `pre-commit run --all-files` clean (ruff lint, ruff format, mypy, codespell, markdownlint, gitleaks)
• `hatch -e docs run mkdocs build --strict` clean
• DCO sign-off

What remains for issue #34

12 more module groups still under `ignore_errors`. Will land in subsequent batches:

```
locus.rag.* locus.memory.* locus.integrations.*
locus.streaming.* locus.loop.* locus.playbooks.*
locus.server.* locus.multiagent.graph locus.hooks.builtin.*
locus.reasoning.* locus.skills.* locus.a2a.*
```