Collapse execute/executeParallel into a shared executeFlow

[kindermax]

Parent

#355

What to build

Extract executeFlow(ctx *Context, runScripts func() error) error containing the four phases that execute() and executeParallel() currently duplicate verbatim: initCmd, executeDepends, persistChecksum, and executeAfterScript. Both existing methods become thin callers that supply either a sequential or a parallel runScripts closure.

No observable behaviour changes. The test suite from #363 must remain fully green as the verification signal.

Acceptance criteria

• A shared executeFlow function (or equivalent internal helper) encapsulates initCmd, executeDepends, persistChecksum, and executeAfterScript
• execute() delegates to executeFlow with a sequential runScripts closure
• executeParallel() delegates to executeFlow with a parallel (errgroup) runScripts closure
• No duplication of the four shared phases between the two callers
• Public interface (Execute, NewExecutor, Context, error types) is unchanged
• All tests from Add RecordingRunner and Execute() test suite #363 pass without modification
• All existing Bats integration tests pass

Blocked by

• Introduce ScriptRunner seam into executor #362 (ScriptRunner seam simplifies the runner calls inside executeFlow)
• Add RecordingRunner and Execute() test suite #363 recommended first — its test suite provides the regression signal that verifies this refactor is safe