Collapse :add and :install into one verb (0.2.0)

[jamilabreu]

Whether a package ships an Igniter installer is upstream's business and
changes over time, but the two-verb API made every workflow file encode
it: a package gaining an installer meant hand-editing {:add, :x} to
{:install, :x} in every workflow naming it.

That distinction only existed because installs had to be queued into a
subprocess, which in turn rested on the belief that installers cannot be
composed mid-run. They can — Igniter's own run_installers/5 composes them
into the same igniter; igniter.install just ends with do_or_dry_run
because it is a top-level task.

So installs now happen in-run, using public Igniter API: collect the
install packages up front, add and fetch them once via
apply_and_fetch_dependencies/2 (which writes only the deps change,
leaving the rest of the diff pending), then compose each .install at
its position in the step list.

With installs composable, the verbs collapse: {:add, name} runs Starter's
step when it has one and otherwise installs the package and runs its own
installer. {:install, name} survives as the explicit override.

Consequences:

• One diff, one confirmation. No queued subprocess for installs, so the
  --yes-into-subprocess workaround (0.1.1) and the argv-leak bug (0.1.2)
  are structurally impossible for installs.
• Ordering is list position. A step after one that installs a package
  sees its effects, so sort_deps is an in-run step again and oban_pro
  just follows oban. This supersedes 0.1.2's queued sort_deps.
• Runs print a plan naming what each step resolved to, which is where
  the built-in-vs-installer distinction lives now that the step list no
  longer draws it.

The fall-through means a typo'd step name would become a doomed Hex
lookup, so names closely resembling a built-in step raise a suggestion
instead.

Dependencies remain the one thing that reaches disk before the main
diff — an installer has to be on disk to compose at all. That change is
shown and confirmed on its own.

Verified against a real project: oban.install runs in-run, and sort_deps
placed last sorts the deps the installers added.

---

Note

High Risk
This rewrites core workflow execution (dep prefetch timing, single diff vs subprocess installs) and changes the public step semantics users encode in workflow files, though {:install} is preserved.

Overview
0.2.0 collapses package setup into {:add, name}: built-in Starter steps when they exist, otherwise add the dep and compose the package’s <name>.install inside the same Igniter run. {:install, name} remains as an explicit “always use upstream’s installer” override.

Starter.Runner no longer queues mix igniter.install after apply. Install packages are collected up front, written to mix.exs, and fetched via Igniter.apply_and_fetch_dependencies/2 (separate confirm), then each installer runs at its list position in one combined diff. Runs print a Starter plan of how each step resolved, and Starter.Runner.installs/2 exposes the prefetch package set.

mix starter.new’s catalog now uses {:add, :oban} (etc.) with oban_pro after oban, and sort_deps as an in-run {:gen} step instead of a post-install queue. {:add} typos near built-in names raise “Did you mean?” instead of Hex lookups.

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