Architecture: layering, coupling, and structural findings

[jongio]

Findings from architecture review, smells detection, and code quality analysis of the TUI and module structure.

Checklist

• config package imports tui/styles (layering violation) (refactor(config): config package imports tui/styles — layering violation #35)
• copilot.Client depends on concrete *data.Store instead of an interface (refactor(copilot): Client depends on concrete *data.Store instead of an interface #36)
• UI components import internal/data directly, missing view-model layer (refactor(tui): UI components import internal/data directly — missing view-model layer #37)
• model.go is 3800+ lines, needs splitting into focused files (refactor(tui): model.go is 3800+ lines — split into focused files #38)
• SessionList and PreviewPanel are god objects with 54 and 34 methods (refactor(tui): SessionList and PreviewPanel components are god objects with 54 and 34 methods #42)
• platform.ShellInfo and TerminalInfo leak as concrete types across UI layers (refactor(tui): platform.ShellInfo and TerminalInfo leak as concrete types across UI layers #43)
• Extract 600-line Update method into per-message handler functions (refactor(tui): extract 600-line Update method into per-message handler functions #77)
• Decompose 62-field Model struct into focused sub-models (refactor(tui): decompose 62-field Model struct into focused sub-models #78)

Notes

Items #38, #77, and #78 are closely related. Splitting model.go, decomposing Model struct, and extracting Update are parts of the same refactoring effort. Tackle them together.

Supersedes: #35, #36, #37, #38, #42, #43, #77, #78