[wowi42]

Implements item 2 of Control flow and duplication from #186:

Centralize agent rebuild in one rebuild_agent(session, client, context, ...) helper used by start_main_run, slash commands, and mid-turn compaction.

What changed

Every main-agent rebuild path hand-rolled the same completion_model / resolve_temperature / resolve_extra_body + build_agent preamble (~15 copies across app, event_handler, and slash commands).

Introduced AgentBuildCtx (src/ui/state.rs): a borrow bundle of the build inputs with a single rebuild_agent(model_id, reasoning_enabled) method, obtainable from UiContext::agent_build_ctx() (main loop, mid-turn compaction) and SlashCtx::agent_build_ctx() (slash commands), or built from owned clones in the startup prebuild task.

Converted call sites:

• ensure_agent — used by start_main_run, mid-turn compaction, worktree-merge run
• handle_compress — /compress and auto/mid-turn compaction
• SlashCtx::rebuild_agent / rebuild_agent_with_client / switch_to_prompt_model, apply_prompt_model — all slash-command rebuilds (/model, /provider, /add, /mode, prompt-model switches, ...)
• /loop respawn and worktree-return respawn (event_handler.rs)
• worktree startup (x2), worktree exit, MCP reconnect, background prebuild (app.rs)
• apply_model / handle_model (slash/providers.rs)

Net: −312 / +149 lines. Out of scope and untouched: subagent and /btw builders, non-TUI one-shot/ACP modes.

Behavior preserved

• advisor global-state syncs keep their original order at slash sites
• /loop and worktree respawns keep hardcoded reasoning = true
• model switches still build against the not-yet-committed model id
• MCP lazy-connect stays at the startup sites

Verification

• cargo fmt
• cargo test — 608 passed (default features)
• cargo test --features advisor,hooks,memory — 784 passed
• cargo test --no-default-features — 510 passed
• cargo install --path . --debug OK

Refs #186