feat: OpenAI, Anthropic, and native-Ollama providers + media pipeline

Phase 3:
- provider/openai: Chat Completions for OpenAI + compat endpoints (SSE
  streaming with by-index tool-call assembly, response_format json_schema,
  legacy max_tokens option, reasoning_effort)
- provider/anthropic: Messages API (tool_use/tool_result, GA structured
  output via output_config.format, full SSE event parser, 529 transient)
- provider/ollama: one native /api/chat client behind the ollama,
  ollama-cloud, and foreman built-ins (presets; NDJSON streaming tolerant
  of foreman's buffered single-object responses; object tool arguments;
  format-schema structured output; think mapping)
- media/: capability normalization (sniff, downscale, transcode, byte
  ladder, ErrUnsupported), wired into the chain executor per target with
  penalty-free advance past incapable elements
- registry: real provider + scheme wiring, WithHTTPClient option, required
  env-foreman TLS chat round-trip test
- ADR-0009 multimodal strategy, ADR-0010 tools/structured mapping; README
  matrix + CLAUDE.md synced

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/adr/0010-tools-structured-output-mapping.md

@@ -0,0 +1,49 @@
+# ADR-0010: Tools and structured output — one canonical shape, native mappings
+
+**Status:** Accepted — 2026-06-10
+
+## Context
+
+Tool calling and schema-constrained output exist on every target but with
+different wire shapes (verified against current docs, June 2026; shapes
+recorded in each provider's package doc). The canonical API must hide all
+of it.
+
+## Decision
+
+Canonical: `Tool{Name, Description, Parameters (JSON Schema), Handler}`;
+`Response.ToolCalls[]{ID, Name, Arguments json.RawMessage}`; results return
+as `ToolResultsMessage(ToolResult{ID, Name, Content, IsError})`. Structured
+output via `WithSchema(schema, name)`. Per-provider mapping:
+
+| Concern | OpenAI(+compat) | Anthropic(+compat) | Ollama/foreman | Google (Phase 4) |
+|---|---|---|---|---|
+| Tool def | `tools[].function{name,description,parameters}` | `tools[]{name,description,input_schema}` | `tools[].function` | `FunctionDeclaration.ParametersJsonSchema` |
+| Call args | JSON **string** → RawMessage | `tool_use.input` object | `arguments` **object** | `FunctionCall.Args` map |
+| Results | one `role:tool` msg per result (`tool_call_id`) | one **user** msg, `tool_result` blocks (`is_error` native) | `role:tool` + `tool_name` | `FunctionResponse` parts |
+| IsError | `"ERROR: "` content prefix | `is_error: true` | `"ERROR: "` prefix | response payload field |
+| Forced choice | `tool_choice` string / named object | `{"type":"any"/"tool"/"none"}` | none → drop tools; others best-effort ignored | `FunctionCallingConfig` modes |
+| Structured | `response_format json_schema` (no strict flag) | `output_config.format json_schema` (GA mechanism) | `format: <schema>` | `ResponseJsonSchema` + JSON MIME |
+
+Cross-cutting decisions:
+
+- **Missing call ids are synthesized** (`call_<n>`) — Ollama and some
+  compat servers omit them; the agent loop needs ids to match results.
+- **Streaming buffers tool-call arguments to completion** (ADR-0002):
+  OpenAI fragments accumulate by index, Anthropic `input_json_delta`
+  fragments accumulate per block; consumers only ever see parseable calls.
+- **No strict-mode flag is sent** to OpenAI: strict mode imposes schema
+  constraints (every property required, additionalProperties:false) that
+  caller-supplied schemas may not satisfy. The `Generate[T]` reflector
+  (Phase 5) emits strict-compatible schemas anyway.
+- `SchemaName` feeds providers that need a name (OpenAI; default
+  "response"); others ignore it.
+- Tool handlers never panic the loop: `Toolbox.Execute`/`ExecuteTool`
+  recover panics and JSON-encode results (ADR to agent loop, Phase 5).
+
+## Consequences
+
+- One test matrix per provider asserts the exact wire JSON both directions;
+  drift is caught by httptest fixtures, not in production.
+- Ollama's missing tool_choice means "required" cannot be enforced there —
+  documented in the README matrix rather than emulated.