fix(deps): break :application_controller cycle with vfs at boot

[ivarvong]

Summary

• Add runtime: false to the :vfs dep so Mix does not auto-inject it into exgit's generated applications list.
• Add plt_add_apps: [:vfs] to the dialyzer config to keep vfs beams in the PLT after runtime: false removes them from the default closure.
• Add Exgit.BootWithVfsTest — a regression smoke test that boots :exgit and :vfs together in the same VM under a 30s per-test timeout.

The bug

Bundling both :exgit and :vfs into the same release (e.g. a downstream agent app) deadlocked :application_controller at boot.

Mechanism:

1. :exgit had {:vfs, ..., optional: true} — Mix still adds optional deps as application edges.
2. At boot, :application_controller walks applications for every loaded app. When it sees :vfs listed by :exgit, it tries to start :vfs.
3. :vfs (in dev/test) depends on :exgit for its own integration tests against exgit as a vfs storage backend.
4. Cycle in :application_controller → deadlock.

optional_applications only tells the controller "it's OK if this app is absent" — it does not break the dependency edge when the app is present. Once both libs are in the same build, you wedge.

The fix

runtime: false on the :vfs dep is the load-bearing flag. It tells Mix "compile against this, but don't list it in applications." That is exactly the relationship we want: exgit uses vfs's protocol module at compile time (for defimpl VFS.Mountable, for: Exgit.Workspace) but does not depend on a vfs supervision tree at runtime. Same shape as Phoenix's optional templating adapters.

Architecture invariant preserved (per CLAUDE.md):

Backend libraries take :vfs as an optional dep — never the reverse. Dependency direction is backend → vfs.

• exgit standalone: works without vfs ever entering the picture.
• exgit + vfs together: defimpl compiles in, protocol consolidation picks it up, Exgit.Workspace is mountable.
• vfs → exgit: only in vfs's own dev/test env, for testing exgit as a vfs storage backend. Production consumers of vfs that want git-backed mounts add exgit themselves.

Verification

Locally on darwin, Elixir 1.19, OTP 28:

Check	Result
mix format --check-formatted	clean
mix compile --warnings-as-errors	clean
MIX_ENV=dev mix credo --strict	0 issues / 179 files
MIX_ENV=dev mix dialyzer	0 errors
mix test --warnings-as-errors	699 tests, 0 failures (incl. new smoke test)
mix test test/exgit/workspace_vfs_test.exs --include vfs	34 tests + 3 properties (full vfs conformance against Exgit.Workspace)
Reproducer: mix run -e 'Application.ensure_all_started(:exgit); Application.ensure_all_started(:vfs); IO.puts("ok")'	prints ok (previously deadlocked)

Inspected _build/dev/lib/exgit/ebin/exgit.app:

• applications no longer contains vfs
• optional_applications is []
• defimpl modules (Elixir.VFS.Mountable.Exgit.Workspace, Elixir.Exgit.Workspace.VFS) remain in modules — exactly the intended compile-time-only integration shape.