pikaci: fast-path staged Linux runs on single-host Incus

[justinmoon]

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📥 Commits

Reviewing files that changed from the base of the PR and between 027b2aa and 4cca428.

⛔ Files ignored due to path filters (2)

• nix/ci/pika-core-workspace/Cargo.lock is excluded by !**/*.lock
• nix/ci/rmp-workspace/Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (12)

• crates/pikaci/src/executor.rs
• crates/pikaci/src/main.rs
• crates/pikaci/src/model.rs
• crates/pikaci/src/run.rs
• crates/pikahut/tests/guardrails.rs
• docs/incus-migration-plan.md
• flake.nix
• nix/ci/linux-rust.nix
• nix/ci/rmp-workspace/rmp-template/desktop/iced/Cargo.toml
• nix/ci/rmp-workspace/rmp-template/rust/Cargo.toml
• nix/incus/pikaci-image.nix
• scripts/check-cargo-machete

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 6 additional findings in Devin Review.

[devin-ai-integration]

🟡 forced_remote_linux_vm_backend silently ignores unknown env var values instead of failing

When an operator sets PIKACI_REMOTE_LINUX_VM_BACKEND to an unrecognized value (e.g., a typo like "Incus" or "mcrovm"), the _ => None catch-all silently falls through to auto-detection logic rather than returning an error. This is inconsistent with the analogous remote_linux_vm_incus_mode() at crates/pikaci/src/executor.rs:2264-2276 which explicitly bails on unrecognized values. Since the migration plan at docs/incus-migration-plan.md:862 describes this env var as the "rollback escape hatch," silently ignoring a typo could cause an operator to believe they've forced microvm rollback when auto-detection is actually selecting Incus.

Prompt for agents

In crates/pikaci/src/model.rs, the forced_remote_linux_vm_backend function at line 569-578 has a catch-all _ => None arm that silently ignores unrecognized PIKACI_REMOTE_LINUX_VM_BACKEND values. Change the _ => None arm to log a warning or return an error (bail!/panic) to match the fail-fast pattern used in remote_linux_vm_incus_mode() at crates/pikaci/src/executor.rs:2264-2276. For example, change it to something like:

_ => {
    eprintln!("warning: unsupported {} value {:?} (expected 'incus', 'microvm', 'auto', or empty); falling through to auto-detection", REMOTE_LINUX_VM_BACKEND_ENV, raw);
    None
}

Or, for a stricter approach matching the existing pattern, make the function return anyhow::Result<Option<RemoteLinuxVmBackend>> and bail on unknown values.

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