refactor: replace SessionManager.Create positional args with opts struct

[d0ugal]

Summary

SessionManager.Create() currently takes 14 positional string/bool parameters, making it error-prone and requiring every caller to be updated when a new option is added (as seen in #367 / #374).

Proposal

Replace the positional parameters with a CreateOpts struct:

type CreateOpts struct {
    Name            string
    AgentName       string
    RepoPath        string
    BaseBranch      string
    Prompt          string
    Model           string
    ParentID        string
    NoRepo          bool
    ShareWorktree   string
    AgentHooks      bool
    InPlace         bool
    AllowConcurrent bool
    Rows            uint16
    Cols            uint16
}

Benefits:

• Self-documenting at call sites
• New options use zero-value defaults — existing callers don't break
• Harder to swap two adjacent string or bool params by accident

[d0ugal]

Still valid. SessionManager.Create still takes 14+ positional args (internal/daemon/daemon.go:~340: name, agentName, repoPath, baseBranch, prompt, model, parentID, noRepo, shareWorktree, agentHooks, inPlace, allowConcurrent, skipModelValidation, rows, cols, envExtra…). No CreateOpts struct yet. Refactor would touch ~12 call sites but removes positional-swap risk.