Simplify session open preparation flow

[thymikee]

Summary

The open refactor in PR #268 left src/daemon/handlers/session-open-prepare.ts as the most abstraction-heavy part of the session handler split.

The current shape still uses:

• PreparedOpenCommand
• { response } | { prepared } discriminated-union returns
• prepareResolvedOpenCommand(...) as an intermediate step before completeOpenCommand(...)

That keeps the module readable, but it also adds indirection and parameter threading that do not appear to buy much now.

Goal

Reduce session-open-prepare.ts by collapsing the intermediate prepared-command layer.

A likely direction is:

• let prepareExistingOpenCommand(...) and prepareNewOpenCommand(...) complete the open flow directly, or
• otherwise remove the PreparedOpenCommand return object and keep the shared logic in a smaller helper that returns only the concrete values still needed

Scope

In scope:

• src/daemon/handlers/session-open-prepare.ts
• src/daemon/handlers/session-open.ts
• src/daemon/handlers/session-open-surface.ts if needed

Out of scope:

• changing CLI behavior
• changing runtime-hint semantics
• changing macOS surface behavior

Notes

This should be treated as structural cleanup, not a behavior change.

The intent is to keep the module split from PR #268, but remove the remaining abstraction overhead in the open path.

[thymikee]

Closed by #296