feat: add coding agent workspace

[sionic-khope]

Summary

• replace the expanded terminal surface with a single speech-bubble coding workspace
• keep the enlarged Miku companion on the right and return-to-idle controls intact
• add provider and approval selection plus an embedded rectangular terminal drawer
• route Codex and Claude prompts to the local PTY with quoted commands and explicit permission mappings

Verification

• export DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer && swift build -Xswiftc -warnings-as-errors
• export DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer && swift test -Xswiftc -warnings-as-errors (144 tests)
• MIKUCODE_CAPTURE_DIR=/tmp/mikucode-agent-capture swift test -Xswiftc -warnings-as-errors --filter TerminalVisualSmokeTests/testIdleOverlayAndActiveTerminalRenderAtDisplaySize
• git diff --check

Notes

• no external Terminal.app launch path or second desktop window was added
• Codex and Claude are available on this workstation; Pi and OpenCode remain selectable and report through the embedded terminal if their CLIs are installed