Rename Motoko entry points app.mo → main.mo (align with icp new templates, mops, and skills)

[marc0olo]

Problem

Motoko entry points are inconsistent with the rest of the ecosystem, and the name app.mo is arbitrary for many examples. Current census (tracked files):

• 34 examples use backend/app.mo (or <role>/app.mo in multi-canister examples, e.g. parallel_calls/caller/app.mo)
• 1 example (motoko/who_am_i, the legacy-layout II reference example) already uses main.mo

Recommendation: rename to main.mo (lowercase), not Main.mo

Main.mo (PascalCase) was considered for consistency with the repo's module-file convention (Counter.mo, Types.mo), but every official source of truth uses lowercase main.mo:

Source	Entry file
icp-cli-templates (motoko template, icp new)	src/main.mo
icp-cli-templates (hello-world/motoko-backend)	src/main.mo
mops docs/skill ([canisters] snippets)	main.mo
Motoko skill architecture (types.mo / lib/ / main.mo)	main.mo
Caffeine project layout	main.mo
motoko/who_am_i in this repo	main.mo

A developer graduating from icp new to these examples should see the same filename. The PascalCase convention applies to importable modules named after the type they export; the entry point is a composition root, not an imported module, and the ecosystem treats it as the special lowercase main.mo (analogous to lib.rs/main.rs in Rust — which our Rust examples already follow).

Migration scope

1. git mv backend/app.mo backend/main.mo in 34 examples (incl. <role>/app.mo in parallel_calls, pub-sub, etc.)
2. Update main = "backend/app.mo" in each mops.toml [canisters.*] section
3. Update AGENTS.md: canonical structure tree, mops.toml snippet, and the naming-conventions bullet ("Entry point file: always backend/app.mo")
4. Sweep READMEs/scripts for literal app.mo references
5. No CI changes needed (paths resolve via mops.toml); ICP Ninja sync picks up the rename via the normal mirror

Mechanical, single-PR-able, and zero behavioral risk (mops check across all examples verifies).