Tracking: adopt the proposed development model

[krystophny]

Umbrella for the proposed development model (docs/development-model.md,
PR #54). For group discussion on Discord before adoption.

Done in PR #54:

• Relocate infra to $CODE/infra; external workspace-shared at $CODE/external
• Retire the umbrella build-everything CI (main.yml)
• Design note + workflow templates (docs/templates/)

Remaining:

• Standardize <DEP>_BRANCH across consumers (Standardize <DEP>_BRANCH override across libneo consumers #55)
• libneo release workflow + downstream gate (Release workflow: release branch + downstream reverse-dependency gate libneo#277)
• Add integration.yml to each consumer (template: docs/templates/downstream-integration.yml)
• Adopt YY.MINOR.PATCH, tag-derived, no leading zeros, across the codes
• Decide setup.yml fate (Decide fate of setup.yml (release-tarball builder) #56)

Model in one line: a self-describing find_or_fetch graph, per-code CI at the
source, and release-time reverse-dependency validation owned by the upstream.