refactor!: reorganize crate into domain modules and simplify LLM client API

[tsharp]

refactor!: reorganize crate into domain modules and simplify LLM client API

BREAKING CHANGE: removes the ChatClient wrapper struct, deletes all
src/bin/* demos, and moves several top-level modules into directory-based
submodules. Public re-exports are preserved where possible, but downstream
code constructing ChatClient or importing from the old paths must update.

• refactor(llm)!: replace ChatClient wrapper with pub type LlmClient = Arc<dyn GenericChatClient>; agents now take the client via
  Agent::new(cfg).with_client(client) instead of with_llm_client
• refactor(tools): consolidate tool.rs, tools/, and tool_loop_detection.rs
  into a single tools/ module (registry, native, mcp,
  loop_detection, builtin)
• refactor(workflow): split workflow.rs + step.rs + step_impls.rs into
  workflow/{mod,step,steps/{agent,transform,conditional,subworkflow}}
• refactor(context): split context.rs + context_strategies.rs into
  context/{mod,strategies/{token_budget,sliding_window,summarization, message_type}}
• refactor(runtime): group runtime.rs, retry.rs, and timeout.rs under
  runtime/
• refactor(agent,event): convert agent.rs and event.rs to directory
  modules with co-located tests.rs
• fix(event): rename EventStream::from_offset -> get_from_offset to
  satisfy wrong_self_convention
• chore: bump workspace to v0.4.0, add clippy.toml, Makefile.toml,
  rust-toolchain.toml
• chore: drop src/bin/* demos (moved/retained via crates/agent-discourse)
• docs(readme): rewrite as a concise crates.io-style overview with examples
  matching the new API
• test: update all integration tests and benches to the new client API