Problem

When a Project.toml declares a [sources] table (path/url deps — common in
monorepos and lib/* workspaces), that dep is satisfied in-tree by the
checkout, not from the registry. julia-actions/julia-downgrade-compat rewrites
every dep's [compat] to =<floor>, but the in-tree path version is whatever
the checkout is — so the pin can't be satisfied and resolution dies with:

ERROR: Unsatisfiable requirements detected for package X:
  ... no versions left

before any test runs. This is the org-wide [sources]+[extras]+[compat]
downgrade breakage.

Fix

Treat every [sources] dep the same way stdlibs are already treated: never
downgrade-pin them, by unioning their names into the effective skip list.

downgrade.yml

In the existing "Compute effective downgrade skip list" step, for each dir
in projects (comma-separated, default .), parse <dir>/Project.toml's
[sources] keys and union them into EFFECTIVE_SKIP:

using TOML
get(TOML.parsefile(joinpath(dir, "Project.toml")), "sources", Dict()) |> keys

Missing Project.toml / missing [sources] section are guarded; the found
source names are echoed. EFFECTIVE_SKIP is now stdlibs ∪ [sources] ∪ inputs.skip.

sublibrary-downgrade.yml

Its skip step already unions stdlibs + in-repo lib/* directory basenames.
Added: parse the downgraded sublibrary's own [sources] keys
(matrix.project/Project.toml) and union them in. The basename skip only
matches deps whose source name equals a lib/ dir name; parsing the actual
[sources] keys also catches url-/aliased sources.

README.md

Documents that [sources] path/url deps are auto-skipped for both workflows.

Local verification

• actionlint clean on both workflows.
• Unit-tested the parse logic against throwaway Project.toml fixtures (with
  [sources], without [sources], a missing dir, and a sublibrary's own
  [sources]). Extracted-step-body output:

[sources] deps in proj_a: MyURLDep,MyPathDep
No Project.toml in 'no_such_dir', skipping [sources] scan
Effective downgrade skip: Pkg,TOML,Statistics,MyURLDep,MyPathDep,ExtraSkipDep

[sources] deps in lib/SubLib: SiblingLib
Effective downgrade skip: Pkg,TOML,Statistics,SubLib,SiblingLib

NOTE

v1 must be retagged after merge for callers (which pin @v1) to pick this
up.

Ignore until reviewed by @ChrisRackauckas.