ci: fix Rust caching and cut nightly build time

[Ziinc]

Nightly desktop builds were taking 17–27m per target and Android 21m. Every Rust job in the nightly compiled from scratch.

Root causes

• The Rust cache never worked. refactor: remove Mission Control and dead terminal code; key agent columns "agent-<id>" #629 added a root workspace, so build output goes to ./target. nightly.yml, release.yml and remote-e2e.yml still cached src-tauri/target. Logs show No cache found and a 178 MB upload that holds only the cargo registry.
• The cache key never changed. rust-cache hashed the orphaned src-tauri/Cargo.lock. Cargo uses the root Cargo.lock.
• Cargo was ignoring the [profile.*] sections. Cargo only reads profiles from the workspace root, and since refactor: remove Mission Control and dead terminal code; key agent columns "agent-<id>" #629 they lived in src-tauri/Cargo.toml. It printed a warning: profiles for the non root package will be ignored. Release builds silently lost opt-level = "z", strip and panic = "abort", which telemetry.rs relies on.
• Each LTO'd crate type gets its own whole-program link. With fat LTO, the lib's staticlib and cdylib outputs each take a full link: about 3.4m for the lib step plus about 3.8m for the bin on Linux.

Changes

• rust-cache: workspaces: ". -> target" in nightly (×3), release and remote-e2e.
• Delete src-tauri/Cargo.lock; fix the README make bump line that mentions it.
• Move the profiles to the root Cargo.toml unchanged. This restores the pre-refactor: remove Mission Control and dead terminal code; key agent columns "agent-<id>" #629 behaviour.
• Desktop nightly and release builds: a CI step rewrites crate-type to ["rlib"] before tauri build. Desktop only needs the bin; mobile and NAPI keep all three crate types.
• Nightly desktop: lto = "thin" and codegen-units = 16 via env. Release keeps fat LTO from Cargo.toml, and its redundant env overrides and stale comment are removed.
• Nightly Android and iOS debug builds compile dependencies at opt-level = 1, set by a CI-written .cargo/config.toml that overrides the manifest's 3.
• Nightly Linux uses --bundles deb,rpm and skips the AppImage. Releases still build all bundles.

Side effects to note

• Moving the profiles also brings back profile.dev.package."*" opt-level = 3 for local dev and ci.yml test builds. Dependencies compile slower on a cold cache, and tests run faster.
• The first run after merge still starts cold, since this PR creates a new cache key. Gains show from the second run on.

Verification

• cargo metadata --locked succeeds against the root lockfile, and the profile warning is gone.
• A toy crate confirmed that .cargo/config.toml overrides profile.dev.package."*" in the manifest.
• The sed + grep crate-type step was tested on a copy of src-tauri/Cargo.toml.
• All edited workflows parse as YAML.
• Not run: a full Tauri build. The container lacks webkit2gtk. A workflow_dispatch of Nightly on this branch is the real check.
  Not addressed here: the macOS x86_64 job failure in run 37449540162.
  Generated by Claude Code