mob.new_plugin: scaffold against current mob, not the abandoned ~> 0.6

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Problem

MobDev.Plugin.Scaffold hard-coded the mob requirement in two places: {:mob, "~> 0.6"} in the generated mix.exs and mob_version: "~> 0.6" in every tier's manifest. Published mob is 0.7.x, so a freshly scaffolded plugin can't activate:

installed :mob 0.7.x does not satisfy mob_version "~> 0.6"

and mix deps.get resolves a stale mob. The literal also lived in five separate template strings, so mix.exs and the manifest could drift apart with nothing catching a stale value before a user hit it.

Reported as #21, surfaced by mob_ci which dogfoods mob.new_plugin --tier 0..4 and had to hand-bump the generated fixtures. Confirmed by a consumer in the issue thread.

Fix

Derive the requirement instead of hard-coding it:

• Scaffold.mob_requirement/1 (pure) maps a concrete version to "~> MAJOR.MINOR", or returns the compiled fallback on nil.
• Scaffold.detect_mob_requirement/0 prefers the version of :mob actually resolved in the project (Application.spec(:mob, :vsn) after Application.load/1), falling back to a single @fallback_mob_requirement ("~> 0.7") when mob isn't loadable (standalone scaffolding). mix mob.new_plugin calls this and threads the result into Scaffold.files_for/3; the templates stay pure.
• One literal (@fallback_mob_requirement); mix.exs and all manifests interpolate the threaded value so they can't disagree.
• The ~> 0.6 example in the manifest validator's error message is bumped to ~> 0.7 for consistency (the mob_dev adopt-deps ~> 0.6 references are left alone — those pin mob_dev itself, which is 0.6.x).

Tests

New Scaffold describe block (#21): asserts the derivation, that every tier's mix.exs and manifest agree, that the default is not ~> 0.6, that files_for/3 honors an explicit requirement, and that a generated manifest validates against a mob version satisfying the default (derived from the requirement, so the check tracks future bumps). mix test test/mob_dev/plugin/scaffold_test.exs → 40 passed. mix format + mix credo --strict clean.

ADR: decisions/2026-06-22-scaffold-derives-mob-requirement.md. CHANGELOG entry under [Unreleased].

Note

When mob's major.minor moves again, @fallback_mob_requirement still needs a human bump — mob_dev doesn't depend on mob, so CI here can't compare against the latest published mob automatically. The test guards against silently shipping the old floor.

Closes #21

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