Delete dead legacy terminal code (AppState, raw-mode functions, duplicate spinners)

[joshuacrass]

Problem:
The codebase contains a layer of legacy pre-BubbleTea code that is no longer reachable from the TUI runtime:

• AppState struct in main.go (comment: "kept for compatibility with legacy code still being migrated" — no migration is in progress)
• renderHeader, renderStatusBar, printIssueDetail, printPRDetail, printIssuesTable in render.go / terminal.go — write directly to stdout via fmt.Printf, which would corrupt the BubbleTea alt-screen buffer if ever called during TUI operation
• withSpinner, startSpinner / stopSpinner in terminal.go — two duplicate spinner implementations, neither used from any BubbleTea Update handler
• confirmAction in terminal.go — calls form.Run() which takes over stdin/stdout

This dead code increases cognitive surface area, creates a silent correctness risk if accidentally invoked during TUI operation, and contains stty calls and clearScreen side effects.

Solution:
Audit every function in terminal.go and the legacy rendering functions in render.go. Delete any that are not reachable from the BubbleTea AppModel.Update or AppModel.View path. Delete AppState.

Acceptance Criteria:

• AppState is deleted
• All unreachable raw-mode functions are deleted
• Both legacy spinner implementations are deleted
• go vet ./... and go test ./... pass after deletion
• No BubbleTea Update/View path calls any function that writes directly to stdout