## Summary
Fixes#16637. (I hit this bug after 11h of work on a long-running task.)
Plugin cache initialization could panic when an already-absolute cache
path was normalized through `AbsolutePathBuf::from_absolute_path`,
because that path still consulted `current_dir()`.
This changes absolute-path normalization so already-absolute paths do
not depend on cwd, and makes plugin cache root construction available as
a fallible path through `PluginStore::try_new()`. Plugin cache subpaths
now use `AbsolutePathBuf::join()` instead of re-absolutizing derived
absolute paths.
Load plugin manifests through a shared discoverable-path helper so
manifest reads, installs, and skill names all see the same alternate
manifest location.
Split plugin loading, marketplace, and related infrastructure out of
core into codex-core-plugins, while keeping the core-facing
configuration and orchestration flow in codex-core.
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Co-authored-by: Codex <noreply@openai.com>
