Tag lib/ subpackages via TagBot matrix

[ChrisRackauckas-Claude]

Ignore until reviewed by @ChrisRackauckas.

Summary

Mirrors the NonlinearSolve.jl TagBot setup so each registered subpackage under lib/ gets its own git tag/release when it's registered. Today the monorepo only carries the top-level OrdinaryDiffEq tag (v7.0.0, etc.), and there are no tags for any of the 56 subpackages in lib/ — which makes diffing between versions of, say, OrdinaryDiffEqCore or StochasticDiffEq tedious (currently requires git blame on each Project.toml version bump, then scrolling through non-squashed commits).

Background

From Slack discussion with Joshua Lampert and Hendrik Ranocha:

Joshua: I maintain several packages [which] heavily depend on subpackages of OrdinaryDiffEq.jl and I often see myself trying to debug failing CI in the packages I maintain, which come from changes in OrdinaryDiffEq.jl. For this I often want to check diffs between versions of subpackages [...] However, due to OrdinaryDiffEq.jl being a monorepo, [...] there are no Git tags or releases of the subpackages I can use [...] Therefore, I usually navigate on GitHub into the subpackage I'm interesting in and check the commit history.

NonlinearSolve.jl already does this (see .github/workflows/TagBot.yml there); Chris confirmed it should be easy to copy the pattern here and chose to keep merge commits (not squash) so bisecting performance regressions to individual commits still works.

What this PR does

• Splits TagBot.yml into two jobs:
  • TagBot-OrdinaryDiffEq — tags the top-level package (existing behavior, just renamed).
  • TagBot-Subpackages — a matrix job that calls JuliaRegistries/TagBot@v1 once per subpackage with subdir: lib/<package>, the same mechanism NonlinearSolve.jl uses.
• Adds the standard explicit permissions: block and workflow_dispatch.inputs.lookback knob copied from the NonlinearSolve.jl workflow.
• The matrix lists all 56 directories under lib/. Verified each has a name/uuid/version in its Project.toml. The matrix is exactly ls lib/ (verified via comm).

Notes

• fail-fast: false so one subpackage failing to tag doesn't cancel the others.
• This only affects future registrations — TagBot will start emitting <Subpackage>-vX.Y.Z tags from the next registration of each subpackage. It will not retroactively tag previously-registered versions.
• Three historical standalone repos (SciML/DiffEqBase.jl, SciML/StochasticDiffEq.jl, SciML/DelayDiffEq.jl) are already archived, so tagging them here from lib/ is safe — there's no other home to conflict with.

Test plan

• Workflow YAML parses (verified locally with yaml.safe_load).
• Manually trigger workflow_dispatch after merge and confirm no errors when there's nothing to tag.
• Confirm the next subpackage registration produces a <Subpackage>-vX.Y.Z tag on the monorepo.