[Architecture][P4] Phase 4: Consolidate CLI product-line architecture

Parent: https://github.com/terraphim/terraphim-ai/issues/520
Related plan: docs/architecture-improvement-plan.md

Goal

Consolidate CLI architecture so terraphim_agent, terraphim_cli, and terraphim_repl share one runtime core and have clear product boundaries.

Deliverables

• ADR defining CLI product-line strategy (single binary with modes vs multiple binaries with shared runtime)
• Shared command runtime crate adopted by retained binaries
• Removal of duplicated command/runtime wiring

Tasks

• Evaluate existing overlap and decide target model via ADR
• Extract common command execution/runtime logic into shared crate
• Migrate binaries to shared core incrementally with compatibility checks
• Document ownership and purpose of each retained binary

Acceptance Criteria

• Clear ownership and purpose for each CLI binary in docs
• Reduced duplicate runtime logic across CLI crates
• Default and feature-enabled CLI builds pass quality gates

Start by reading docs/architecture-improvement-plan.md and comparing the existing terraphim_agent, terraphim_cli, and terraphim_repl crates. The work requires an ADR decision, shared runtime extraction, incremental migration, and documentation of retained binary ownership. Done means duplicate runtime wiring is reduced and default and feature-enabled CLI builds pass their quality gates.