Auto-create dev branch on first Edit; restrict modal to self-managed

[ericpgreen2]

• For Rill-managed projects, EditBranchDialog never opens. With zero owned dev branches, the Edit button auto-creates one named after the email's local part and routes into the edit session. With one or more, it routes to the user's most-recently-updated owned branch.
• For self-managed projects, the same zero-branch auto-create kicks in. With one or more, the existing modal opens (status quo).
• Adds useOwnDevDeployments (TanStack combine) so EditButton and EditBranchDialog share one source of truth for the user's editable dev deployments.
• Adds deriveDefaultBranchName(email) in branch-utils.ts — lowercased for cross-platform safety, periods replaced with hyphens, leading/trailing -./ trimmed.
• Cross-user branch-name collisions (gRPC AlreadyExists) fall back to opening the dialog with the name pre-filled rather than silently appending a suffix.

Preemptive polish on cloud editing while it's still flagged behind cloudEditing.

Closes GEN-728.

Note: the "one dev branch per user on Rill-managed" rule is a UI convention, not a server constraint — rill project deployment create will still let a CLI user create multiple. The UI defensively picks the most-recently-updated owned branch when multiple exist; backend enforcement is intentionally out of scope here.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

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Developed in collaboration with Claude Code

[notion-workspace]

Restrict edit/create branch modal to self-managed Git case only (Rill-managed: auto-created branches; users can only edit their own always-on dev branches)