fix(owner): pin mt-core/llm-cascade rev, remove stray nested owner workspace

[vitaliytv]

Summary

release-owner.yml has failed on the "Build and upload DMG" step on every owner release since owner@0.16.0 (last working release: owner@0.15.0), with:

error: failed to get `mt-core` as a dependency of package `owner v0.1.0 (.../owner/src-tauri)`
failed to load source for dependency `mt-core`
unable to update https://github.com/nitra/mt.git
could not find `Cargo.toml` in `/Users/runner/.cargo/git/checkouts/mt-2ebc8b02ac64519d/3097c91`

Root cause is a workspace-resolution regression, not just a stale pin:

1. Commit 1efb2bd (2026-07-21) consolidated the repo into a single root Cargo workspace and deleted owner/Cargo.toml, which used to declare its own nested [workspace].
2. PR feat(owner): integrate cascading LLM access #17 (67e3744, landed right before owner@0.16.0) re-created owner/Cargo.toml with a [workspace] block. Cargo resolves against the nearest ancestor [workspace], so owner/src-tauri / owner/owner-llm silently started resolving in this orphaned nested workspace instead of the root one — with no committed lockfile for it.
3. Both mt-core (git dep from nitra/mt) and llm-cascade (git dep from nitra/7n-rules) had no rev pinned in Cargo.toml. With no lockfile to anchor them, Cargo re-resolved both to their upstream HEAD on every fresh CI runner. nitra/mt's HEAD has since drifted structurally, so resolution now fails identically on every run.

Changes

• Deleted owner/Cargo.toml (the stray nested workspace) — restores the single-root-workspace invariant from 1efb2bd. owner/src-tauri and owner/owner-llm are already listed as members in the root Cargo.toml, so nothing else needed to change.
• Pinned mt-core to rev = "03050b869db4828b351defe639b6d09a1c963b4c" in app/src-tauri/Cargo.toml and owner/src-tauri/Cargo.toml.
• Pinned llm-cascade to rev = "c47c5248ecb45cd076dbc2558e3c2d04f010ddfe" in owner/owner-llm/Cargo.toml. Matches the existing house convention (see the comment above the tauri-plugin-agent pin in app/src-tauri/Cargo.toml).
• Added required .changes/ entries for app and owner.

Verification

• Reproduced the exact CI error locally with a clean CARGO_HOME before the fix.
• After the fix: cargo metadata and cargo check --workspace both succeed from a clean CARGO_HOME (simulating a cold CI runner), with no stray owner/target/ or owner/Cargo.lock reappearing.

Test plan

• CI green on this PR (rust lint / build jobs)
• After merge, manually trigger release-owner.yml via workflow_dispatch on an existing owner@X.Y.Z tag (or wait for the next release) to confirm the DMG actually builds and uploads
• Confirm the owner-latest updater channel picks up a release newer than owner-v0.15.0

🤖 Generated with Claude Code