carve: force-replace both carved repo mains at the cut

[mfw78]

Decision recorded 2026-07-22, before the M5 carve. Raised by a train planning pass and independently by review of #520.

The two already-populated carve targets have diverged from shepherd and from each other:

• nullislabs/nexum-runtime@ce5e549 and nullislabs/videre-nexum-module@59170fb carry a tracing refactor (including a new install_host_tracing! in nexum-sdk) that exists in no shepherd branch.
• Both carry open dependabot PRs (12 across the two).
• Review of runtime: make the module event type extensible via a generic custom channel #520 confirmed the divergence from the other direction: nexum-runtime's wit/nexum-host/types.wit still contains the hardcoded intent-status(intent-status-update) variant that runtime: make the module event type extensible via a generic custom channel #520 removes, and scripts/check-venue-agnostic.sh was relocated to nexum/scripts/ there, so shepherd-side diffs cannot apply verbatim.

Decision

The real carve force-replaces both mains. The tracing refactor and the dependabot work currently on those repos are not back-ported into shepherd, and are discarded when the carve overwrites nexum-runtime/main and videre-nexum-module/main from the shepherd extraction.

Recorded so this is a deliberate choice rather than a three-way reconciliation discovered at carve time.

Consequences to honour at carve time

• Do not attempt to merge or rebase the existing carved-repo history; the carve is an overwrite.
• Close the 12 open dependabot PRs on those repos as superseded rather than merging them first.
• Anything genuinely wanted from the tracing refactor must be re-authored in shepherd before the carve base is cut. Nothing is currently identified as wanted; raise it here if that changes.
• Fixes landing in shepherd (such as runtime: make the module event type extensible via a generic custom channel #520's) reach those repos only via the carve, not by propagation.

Acceptance criteria

• The carve runbook states the overwrite explicitly.
• The dependabot PRs on both carved repos are closed as superseded at carve time.

[mfw78]

Delivered by the M5 three-repo carve. The monorepo is split into nullislabs/nexum-runtime (L1), nullislabs/videre-nexum-module (L2), and nullislabs/shepherd (L3 bundle); each carved history-preserving (byte-identical tip oracle, full per-crate history), builds standalone on rev-pinned cross-repo deps, and CI is green. The pre-carve tag preserves the monorepo. Cross-repo deps use git rev pins (no version tags per decision); the 14 dependabot PRs were closed as superseded.