refactor(core): decompose agent module into focused submodules

[bug-ops]

Summary

Resolves #622 — decompose the monolithic agent/mod.rs (2602 lines, 142 functions) into focused submodules.

• Rename streaming.rs → tool_execution.rs for clarity (Extract tool execution logic to agent/tool_execution.rs #648)
• Extract message queue logic (QueuedMessage, drain/enqueue/resolve, image handling) to message_queue.rs (Extract message queue management to agent/message_queue.rs #649)
• Extract builder methods to builder.rs
• Extract slash command handlers to commands.rs
• Extract utility helpers (metrics, tokens, cost) to utils.rs
• mod.rs reduced from 2602 → 459 non-test lines (Slim agent/mod.rs to main loop and dispatch (<500 lines) #650)

No public API changes. No logic changes — pure code movement.

7 files changed, +1270 -1190, 2033 tests passing.

Test plan

• cargo +nightly fmt --check passes
• cargo clippy --workspace -- -D warnings passes
• cargo clippy --workspace --features full -- -D warnings passes
• cargo nextest run --workspace --lib --bins — 2033 passed, 9 skipped
• No public API changes verified
• Performance analysis confirms zero regression