feat(DEN-2443): expose native tasks through the canonical zed CLI

[ORESoftware]

Superseded by clean current-main PR #229.

The reusable task command layer, staged zed-task binary, parity tests, documentation, and permanent workflow were salvaged byte-for-byte. Typed zed task routing, module export, flags registry, and README integration were reapplied semantically to current main 9b88ac2a1504b1571e045b57cb7efc5f315482f0.

The successor passed focused runtime, staged/canonical CLI, asdf/global compatibility, flags/README, formatting, and strict Clippy gates before opening. Its exact candidate is 4c708dcece916a81caace9d82083a523a297475c; fresh independent certification will target that SHA.

This PR's 18-commit divergent history is discarded.

[linear-code]

DEN-2443

DEN-2462

[ORESoftware]

Cross-branch coordination: preserve DEN-2725 external dispatch

An unclaimed follow-up is active on agent/den-2725-root-gitops-dispatch to expose the already-merged standalone validator as zed gitops ....

This PR overlaps src/lib.rs and src/main.rs. On its next rebase/composition, please preserve these boundaries rather than reimplementing them:

• pub mod external_subcommands; in src/lib.rs;
• external_subcommands::dispatch(args) after the existing early modular dispatchers and before ordinary typed flag/config initialization;
• child execution without a shell, built-in-name precedence, sibling-first/absolute-PATH resolution, and exact child exit-code propagation.

The task command remains a built-in and must always win over any zed-task-style external executable. Root GitOps help/completion composition is owned separately in src/completion.rs. No task-runtime semantics are being changed by DEN-2725.