docs: add missing warnings for key ADK footguns and workflow gaps

[AaronForinton]

Summary

Addresses the documentation gaps identified from a real end-to-end AI-agent workflow build (Bella Vista reservation assistant) that caused significant lost time. Each change adds a targeted note or warning where the docs were silent on a real failure mode.

Motivation

A systematic audit against a live implementation identified six gaps where the docs' silence or vagueness caused actual blockers — not theoretical ones. Several required digging through source code to resolve.

Changes

Gap 1 — No local runtime

• testing.md: prominent warning that there is no poly serve or local simulator; all execution is in Agent Studio Sandbox
• what-is-the-adk.md: clarify the ADK manages config files and does not execute agents

Gap 2 — API keys are workspace-scoped

• prerequisites.md: warning that poly init lists all projects visible to the key; seeing unexpected projects means the wrong key is in use; also removes a garbled duplicate section from a previous edit

Gap 3 — Platform-provisioned resources cannot be created via ADK

• voice_settings.md, chat_settings.md, agent_settings.md, speech_recognition.md: note on each page that greeting/style prompt/disclaimer/personality/role/ASR settings are provisioned by Agent Studio on project init and can only be updated, not created; pushing them into a project without a matching .agent_studio_config entry fails with NotImplementedError: Create operation not supported

Gap 4 — Don't copy project directories

• anti-patterns.md: new section explaining why copying a project directory to a different project causes push failures (.agent_studio_config IDs, platform-provisioned resources); correct approach is poly init + poly pull

Gap 6 — No poly merge command

• tutorials/build-an-agent.md: note at Workflow 1 Step 10 and Workflow 2 Step 7 that merging requires the Agent Studio web UI; there is no CLI command

Minor gaps

• tutorials/build-an-agent.md: mark chat/ as optional in the project structure diagram
• tutorials/build-an-agent.md: add tip in AI-agent Workflow 2 Step 3 to run poly docs --all immediately after poly pull, before generating any files — without it, a coding agent has no schema context and will hallucinate resource structure

What was not addressed

• poly merge CLI command — this is a code change, not a doc change; documented as a known gap
• Gap 5 (branch --environment 404) — cli.md already partially covers branch environment behavior; the failure mode may be platform-specific and is not clearly reproducible

Test strategy

• N/A (docs, config, or trivial change)

Checklist

• ruff check . and ruff format --check . pass
• pytest passes
• No breaking changes to the poly CLI interface (or migration path documented)
• Commit messages follow conventional commits

Note: prerequisites.md is also touched by #85 (tab rename). Whichever merges second will need a quick rebase.