fix(examples): stop passing the deprecated messages= to Chat

[schloerke]

Second CI blocker on main, independent of #2485 below it in this stack.

shinychat 0.7.0 (2026-09-04) deprecated messages= on chat_ui() / Chat.ui() in favour of greeting=, and made it an outright error on the Chat constructor unless history=False is also passed:

ValueError: `Chat(messages=...)` requires `history=False`: startup messages
can't be recorded by the conversation-history feature. Use the `greeting`
parameter for a startup message, ...

pyproject.toml floors at shinychat>=0.6.0, so CI resolves 0.7.0. Same shape as #2485: a third-party release moved underneath us.

Changes

• api-examples/Chat/{app-core,app-express}.py — messages= → greeting=. These emit a ShinyDeprecationWarning, and tests/playwright/examples/example_apps.py fails any app that emits a warning, so 30 playwright-examples jobs have been red since 2026-09-04.
• Dropped a stray second ui.chat_ui("chat") from the Core example. It rendered a duplicate, empty chat sharing the "chat" id — the bare call came first, so the example showed an empty chat above the real one. Unrelated to the deprecation and long-standing: 4a9ea3f3 (2025-03-27).
• templates/chat/** (5 apps) — azure-openai, langchain, anthropic, ollama, openai all pass messages= to the constructor, so they now raise on startup. Not exercised by CI, so this was invisible: worse than the examples, but silent.
• shiny/.agents/skills/shiny-for-python/references/chat.md — the bundled Agent Skill taught messages= in four places, including the minimal-app snippet. CLAUDE.md requires updating bundled skills in the same PR as the API they document.

Verification

Built both examples' UI under warnings.simplefilter("error") — the exact condition the check enforces — and both construct clean. black, isort, flake8 clean.

Still failing, and deliberately not fixed here

Two playwright-shiny failures share this root cause but need a decision from the chat owner rather than a guess from me. Both are pre-existing on main (confirmed in run 34021260595, 2026-09-06):

1. bookmark/chat/client_state/{app,app-server}.py pass messages= to the constructor, so they hit the ValueError (6 tests). The app deliberately manages its own conversation state via RepeaterClient and bookmarking, and the test asserts expect_messages("Welcome!") — but greeting= renders outside loc_messages (there's a separate expect_greeting), so migrating it changes what the test is actually asserting. The right fix depends on how py-shiny's chat bookmarking should interact with shinychat 0.7.0's new history= feature.
2. components/chat/icon/test_chat_icon.py::test_validate_chat_basic fails on AssertionError: Locator expected to have class 'bi bi-robot' (2 tests). The test asserts the assistant icon on the startup message, which messages= used to create; migrating to greeting= means there is no such message until the user sends one, so the test needs restructuring rather than a parameter swap.