Harden completion paths and history privacy under concurrent sessions

[raiseCatError]

Harden the cumulative command-intelligence implementation before acceptance review:

• Keep nested path components in native completion capture context while retaining local fuzzy filtering and cached replacement metadata.
• Yield during imported-history identity hashing/indexing, not only parsing. Treat incompatible successful Atuin output as a Native fallback.
• Persist independent immutable deletion-ID tombstones so concurrent session frontends cannot overwrite each other’s deletions. Earlier stack JSON deletion IDs remain readable and union with tombstones; no command text is persisted. Existing session journal schema remains additive and unchanged.
• Report leading-space history ineligibility from zsh itself so detached archive/replay paths preserve the privacy boundary.

Dependency: #239; merge after it. Refs #140, #141, #143, #144.

Validation: build, typecheck, benchmark-script types, git diff --check and full suite (714 passed). Focused tests exercise real native nested-path capture, 100k yielding/cancellation, two independent frontend deletion writers, earlier JSON compatibility, malformed Atuin fallback, and real-zsh leading-space/unexported HISTORY_IGNORE eligibility. Import hashing/indexing 100k p50/p95 107.08/121.41 ms total wall time, yielding every 1,024 records. Node 22/26 cumulative CI dispatched separately. Physical terminal QA remains pending.

Verification note: the RTK-filtered final suite command returned exit 1 with an empty redirected log, so it was inconclusive. Repeated using rtk proxy to preserve raw output; all 714 tests passed, with no test assertion failure reported in that raw run.