fix(pipeline): skip an unreadable subdirectory in the control-file walk instead of aborting the whole index

[AmirF194]

Picking up from the thread on #2001: the abort happens after discovery finishes clean, in cbm_pipeline_build_semantic_manifest's own control-file walk.

semantic_manifest_walk_controls recursively walks the whole repo looking for .gitignore/.cbmignore/package-manifest files. When it hits the locked directory and cbm_opendir fails, it returns CBM_NOT_FOUND, which propagates all the way up and aborts the entire index. discover.c's walk_dir already treats the identical failure as "nothing to discover here" and moves on; the control walk had no reason to be stricter; a permission-denied subdirectory just has no control files to contribute.

Fix skips a nested directory it can't open instead of failing the whole walk. The root call still fails closed (depth 0), so pipeline_semantic_manifest_rejects_non_directory_root keeps passing unchanged.

Added pipeline_semantic_manifest_skips_unreadable_subdirectory: a locked sibling next to a readable one, asserts the walk succeeds and still finds the sibling's .gitignore. Needs a non-root run to mean anything, since root ignores the chmod and the check would be a no-op (same reasoning as test_cli.c's existing root-demotion comment); verified that way in a container both ways.

Not fixed here, and worth flagging so it doesn't get lost: #2020 is the separate issue about the generic error hint collapsing distinct failure codes. It's real and independent of this one; I only touched the abort itself.

Fixes #2001

[github-actions]

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Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

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[DeusData]

Thank you for the focused control-file walk change and the pipeline test. We need more time to review how an unreadable directory should affect indexing and exclusion handling before giving a decision.

The review queue is currently full, so detailed feedback may take a little time. We are working through it carefully and appreciate the work you have put into supporting the project. Thank you for your patience.