feat: accept instruction as a list, let flavors append to it

[dgageot]

A flavor could only replace an agent's instruction wholesale: appending even a short block to a long system prompt meant duplicating the entire prompt inside the patch. The concrete case is docker/gordon's skills flavor, which appends a ~10-line <skills> block to a ~130-line base prompt.

What changes

instruction accepts a list of strings. The parts are joined by a blank line, like several instruction_file entries. Normalisation happens in Agents.UnmarshalYAML, which already round-trips each agent through a raw mapping, so AgentConfig.Instruction stays a plain string for every consumer, strict mode and error positions are unchanged, and HCL configs (converted to YAML first) get it for free. Marshalling emits the joined string: the list is input sugar only.

Flavor key+ promotes a scalar base to a one-element sequence before appending, so instruction+: [extra] extends a string instruction instead of erroring. A mapping base is still rejected. A nonsensical promotion (e.g. model+) surfaces as the usual type error at parse time, consistent with "a flavored config is validated like a hand-written one".

The "bump version" hint also fires on type errors. The list form only exists in the latest schema (15); documents pinned to 13 or 14 keep rejecting it, but now with hint: this syntax is supported by config version 15; update the top-level 'version' field (currently 13) instead of a bare cannot unmarshal []interface {} into ... string.

agents:
  root:
    instruction: You are a helpful assistant.

flavors:
  terse:
    agents:
      root:
        instruction+:
          - Answer in one sentence.

With --flavor terse the instruction becomes You are a helpful assistant.\n\nAnswer in one sentence.

Also updates agent-schema.json, the agents/flavors/overview docs, and adds a terse flavor to examples/flavors.yaml.

Validation

• go test ./pkg/config/... ./pkg/teamloader/... ./pkg/oci/... (includes schema-vs-examples and doc-YAML tests)
• golangci-lint run, go run ./lint .
• docker agent debug config examples/flavors.yaml --flavor terse, and on gordon.yaml: the skills flavor rewritten with instruction+ resolves to the same config as the standalone flavor file.

[aheritier]

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