refactor(ts/agent): remove remaining Python-idiom duplicates from the public API #3592

Follow-up to the review of #3397 (comment), which removed four second spellings from the skill module (generateSkillset, positional new Skill(...), SkillSet.length, SkillSet.items()). A scan of the rest of @a2ui/agent's public surface found these, none of which is a duplicate in Python but each of which is a Python idiom with a more usual TS shape:

1. Skill.toString() returns toMarkdown() (src/skill/skill.ts). A port of __str__; two spellings for one thing. Remove.
2. SkillSet[Symbol.iterator] yields keys (src/skill/skill.ts), as a Python dict does, while a JS Map yields [key, value]. Since size/entries() follow Map, iterate like Map too: implements Iterable<[string, Skill]>, iterator returns entries(). keys() stays for callers who want keys.
3. ExpressParser.wrapDecompiledBlocks(string[]) and ExpressDecompiler.wrapDecompiledBlocks (src/inference-formats/express/{parser,decompiler}.ts), a port of wrap_decompiled_blocks, sit next to wrap(RawResponsePart[]). The only non-test callers are inside ExpressPromptGenerator. Make it internal or fold it into wrap.
4. supportsStreaming exists on both Parser (src/parser/parser.ts) and InferenceFormat (src/inference-format.ts, commented "Mirrors Python InferenceFormat.supports_streaming"). Decide which one owns the answer and drop the other, or have one delegate to the other.
5. A2uiRequestProcessor.activeCatalogs (src/processor/processor.ts) vs catalogs on RendererState, ExpressParser, DirectJsonParser, and PromptGenerator. Rename to catalogs for consistency (mirrors Python's active_catalogs).
6. ExpressParser.decompile(messages: AgentToRendererMessage | AgentToRendererMessage[]) accepts a single message where the abstract Parser.decompile and DirectJsonParser.decompile take only an array. Take the array only, matching the other overload removals.

Items 1–2 and 5–6 are small public API changes and need **BREAKING**: CHANGELOG entries; 3–4 can be done without one if the public member is kept and only delegated.

Update blueprints/codebases/typescript/agents/a2ui_agent/codebase.blueprint.md where method names change.