tidy: group CLI modules into llm/github/ast + refresh README

[enekos]

Housekeeping pass on the aatxe CLI crate + the README. No public/crate-API changes.

CLI crate reorganized

The binary's flat src/ had 11 loose top-level modules mixing concerns. Grouped into three documented modules (git tracked every move as a rename, so blame/history survives):

crates/aatxe/src/
  llm/      claude_code · pi_proxy · subprocess_llm · stub_client · gemini_http
  github/   gh_diff · github_http
  ast/      ast_scope · ast_import_extractor
  commands/  adapter/  cli.rs  curator.rs  main.rs

• main.rs drops from 10 mod lines to 3; each group has a mod.rs documenting its members.
• All cross-references rewritten to the new paths (crate::llm::…, crate::github::…, crate::ast::…).
• The six library crates (aatxe-core, aatxe-bench, aatxe-ast, aatxe-council, aatxe-learn, aatxe-evals, aatxe-ui) are untouched — nothing on crates.io changes.

README

• Replaced the misaligned ASCII hero diagram with a Mermaid flowchart — GitHub renders it natively, so it can't drift out of alignment again.
• Stronger opening: value prop + the three-signal trust argument + the council dogfood baseline up front; mythology demoted to a closing aside.
• Fixed stale facts: dropped the outdated "52 tests" claim, corrected the GH client from reqwest → ureq, and listed all 7 crates in Workspace layout (was 4) with the new CLI-internal grouping.

Verification

cargo fmt --check + cargo clippy --all-targets -- -D warnings clean; full cargo test --workspace passes (0 failures).