[Birfy]

Closes #907

runstore.create now turns an OSError from creating the store root or the run directory into a RunStoreError naming the path and the OS reason, and reports a store path that is an existing file as not a directory. The CLI therefore exits 2 with one error: cannot create run store <path>: <reason> line, and --json returns {"ok": false, "error": ...}.

The wrap stays inside create() (no blanket CLI-edge OSError handling), so tests/test_audit_store_durability.py still sees PermissionError for audit-path failures.

Verified on the box: --store /proc/adstore, /etc/passwd, /root/adstore all exit 2 with no traceback, text and --json.

Tests: tests/test_store_create_error.py (fails on main): file path, patched os.makedirs EROFS, run-dir ENOSPC, and demo exit 2 with/without --json. Docs: docs/plugins.md run-store section. Changelog: changelog.d/907-store-create-error.md.

[Birfy]

Verdict: PASS (tip 387e41b)

• runstore.create wraps only the store-root and run-dir makedirs; OSError becomes RunStoreError with path + strerror, file-as-store reports not a directory. No CLI-edge catch, so audit durability tests keep PermissionError (ran green).
• CLI: exit 2, one stderr line, {"ok": false} under --json; checked on /proc/adstore, /etc/passwd, /root/adstore.
• New tests fail on main and pass here; runstore/demo/audit suites green locally. Docs + changelog fragment present.
• risk_gate: T3 (path agentdescent/runstore.py), 84 lines / 4 files: waiting for Danyang's merge click, not auto-merged.