fix: fade tab strip edges when tabs overflow

[vanvasten]

Summary

• When a pane has more tabs than fit, the strip fades the edge that hides tabs, so it is clear there is more to scroll to. Closing tabs while the strip is scrolled no longer leaves it parked past the end. The fallback SwiftUI strip keeps tabs at a readable minimum width.
• Terminal tab titles stay as the live process title. The automatic "PR #N" and ticket-id titles derived from the git branch are removed (second commit, easy to drop if unwanted).

Builds on #432, which already added the readable minimum width, mouse-wheel scrolling and reveal of the selected tab.

Testing

• New testNativePaneTabBarKeepsTitlesReadableAndScrollsSelectedTabIntoView (15 long-titled tabs: minimum width, overflow, selected tab scrolled into view). Updated the panel-title test in WorkspaceUnitTests.
• A Debug xcodebuild compiled the Swift sources; the only local failure was the Ghostty resources copy script (no zig on this machine). Nothing run locally beyond that; CI runs the tests.
• Edge fade and scroll clamp not checked by eye.

Demo Video

• Video URL or attachment: needed from the author (visual change).

Checklist

• Tested via CI and, for UI or behavior changes, a tagged build (./scripts/reload.sh --tag <name>)
• I added or updated tests for behavior changes
• I updated docs/changelog if needed
• I requested bot reviews after my latest commit
• All code review bot comments are resolved
• All human review comments are resolved