Copy prompt batch 4

[bgrenon]

Summary

Enriched Elixir Ecto and Phoenix guides with content from their corresponding copy prompts. This is batch 4 of the copy prompt enrichment project (batch 1: #4692, batch 2: #4700, batch 3: #4709).

Changes:

• Elixir Ecto: Added pool_size: 10 to config example, Elixir 1.15+ version requirement (harness finding), "Notes for AI-assisted setup" <details> section (:extra_applications anti-pattern, SSL mandatory, transaction guidance, config.exs vs runtime.exs credential handling)
• Phoenix: Added Elixir 1.15+ and Erlang/OTP 25+ version requirements (harness finding), "Notes for AI-assisted setup" <details> section (SSL mandatory, app name handling, mix phx.new is interactive, security cross-reference)

Automated test harness results:

Guide	Score	Key issue
Elixir Ecto	6.5/10	Container's Elixir 1.14 too old for postgrex 0.22+. Agent spent ~14 turns installing newer Elixir. Guide steps followed correctly once environment was set up.
Phoenix	5/10	Same Elixir version wall. Agent hit 60-turn cap while still setting up runtime. Connection verified but mix ecto.create never completed.

Both guides hit the same root cause: the container shipped Elixir 1.14 but current Postgrex requires 1.15+. Added version requirements to both guides based on this finding.

Notable finding: When the guide body shows hardcoded credentials and the <details> section says "don't hardcode for production," agents follow the guide body's code example. The <details> section is effective for corrections that don't contradict the guide's own examples, but cannot override them.

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Project	Deployment	Actions	Updated (UTC)
neon-next	Ready	Preview, Comment	Apr 6, 2026 1:23pm