fix(pty): emit BackTab sequence for Shift+Tab

[infCraft]

Summary

• Preserve plain Tab as 0x09.
• Emit the standard BackTab sequence (ESC [ Z) for Shift+Tab.
• Add regression coverage for the key translation.

User-visible behavior

Shift+Tab can now switch Claude Code modes inside ProGhostty.

Layer touched

• PTY / sessions
• VT bridge / terminal state
• Renderer
• Workspace
• Settings
• Docs / CI

How I tested

• Swift syntax check
• scripts/check-architecture.sh
• Full swift test — blocked locally while building libghostty-vt
• Manual application test

Fixes #3

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Pull request overview

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Comments suppressed due to low confidence (1)

Tests/ProGhosttyCoreTests/PTYLaunchTests.swift:42

• This adds coverage for Shift+Tab, but the PR summary also calls out preserving plain Tab as 0x09; there isn’t a regression test asserting that behavior. Adding an explicit plain-Tab test would protect against future changes that accidentally map Tab into the BackTab sequence.

  @Test func shiftTabProducesBackTabSequence() throws {

[freecodetiger]

well

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