bug: merging a stacked branch fails after its parent branch is deleted

[azimafroozeh]

What happened

g forks from f, main fast-forwards to f, and f is deleted. Merging g into main then fails with branch 'f' not found; expected merged, leaving z and q on main, since everything f committed is also in g and main. Found while testing the fix for #773.

Steps to reproduce

--- schema
node Person {
    name: String @key
}

--- seed

--- mutate
branch create f

--- expect ok

--- mutate branch: f
query insert_z() {
    insert Person { name: "z" }
}

--- expect affected: nodes=1 edges=0

--- mutate
branch create g from f

--- expect ok

--- mutate branch: g
query insert_q() {
    insert Person { name: "q" }
}

--- expect affected: nodes=1 edges=0

--- mutate
branch merge f

--- expect outcome: fast_forward

--- mutate
branch delete f

--- expect ok

--- mutate
branch merge g

Version

omnigraph 0.11.0, main @ 80406fe, built from source (Lance 11.0.0)

Environment

macOS (Darwin 25.5) arm64, local filesystem store (the .gqt harness runs the embedded engine). Not tried on an object-store backend.

Logs / output

branch 'f' not found