fix: correct stale .env.example references to the make dev auto-create flow

[Jacob-9909]

What

The repo references a .env.example file that doesn't exist anywhere in the tree (git ls-files / find confirm it's not tracked or present). The README was refactored so make dev auto-creates a local .env (the validate-dev-env target in makefiles/Makefile), and the top-level README no longer mentions a template to copy — but six stale references were left behind:

• CLAUDE.md — Setup step 1: "Copy .env.example to .env"
• backend/CLAUDE.md — "Required env vars (see .env.example)"
• backend/README.md — cp .env.example .env
• makefiles/Makefile — three error messages (convex-env, convex-push, seed-public-datasets): Error: .env not found. Run: cp .env.example .env

Why

A new contributor following the setup instructions, or running make convex-push / make seed-public-datasets standalone before make dev, hits an instruction telling them to copy a file that doesn't exist.

Change

Point every reference at the actual flow: make dev auto-creates a local .env. Docs-and-strings only — no behavior change.

Verification

• grep -rn "env.example" over the tree returns no matches after the change.
• make -n parses the Makefile cleanly.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 197b88ed-fe4d-4123-8533-1bd5b7cafd67

📥 Commits

Reviewing files that changed from the base of the PR and between 839831c and ac7764c.

📒 Files selected for processing (4)

• CLAUDE.md
• backend/CLAUDE.md
• backend/README.md
• makefiles/Makefile

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📝 Walkthrough

Walkthrough

All references to the manual cp .env.example .env setup step have been replaced across the repository. CLAUDE.md, backend/CLAUDE.md, and backend/README.md now state that running make dev auto-creates the local .env file on first run, directing users to fill in that file directly. The error messages emitted by the convex-env, convex-push, and seed-public-datasets Makefile targets when .env is absent have been updated to match this new guidance.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: correcting stale .env.example references to point users to the make dev auto-create flow.
Description check	✅ Passed	The description is comprehensive and directly related to the changeset, explaining what was changed, why it was needed, and how it was verified.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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