refactor: split aura into aura-cli and aura-ui crates

[Rfluid]

Summary

Splits the aura binary crate into three crates. No behavior change.

Crate	Kind	Contents
crates/aura	binary aura	Thin main.rs: parse args, then aura_cli::dispatch or aura_ui::run
crates/aura-cli	lib	Former src/cli/*. No UI deps; gpui is not in its dependency tree
crates/aura-core	lib	Unchanged
crates/aura-ui	lib	Former app, tray, platform and other UI modules. Former main.rs body is now pub fn run(version)

Code changes beyond moves

21 of the 25 moved source files are identical apart from comments. The rest:

• Version plumbing. env!("CARGO_PKG_VERSION") inside a library crate would now report that crate's version. The binary passes its own version down:
  • Cli::parse_with_version: same parse and exit-on-error behavior as clap's parse(), with the version overridden.
  • aura_ui::run(version) stores it in a OnceLock. The updater and the modal's "check updates" label read it through crate::app_version().
• SIGPIPE reset moved from main into aura_cli::dispatch. It still runs at the same point: after parsing, before the subcommand, before any thread starts.
• The Windows windows_subsystem attribute stays on the binary.
• Manifests: each crate lists only the deps it uses. Nothing added or bumped.

Per-crate versioning and scripts/prepare-release.sh are unchanged. The new crates start at 0.1.0. build-macos-app.sh, the justfile and cargo run -p aura still work because the aura crate stays at crates/aura.

The PR also updates the crates/aura/... paths in current docs (docs/, .agent/context, .agent/skills, .design/, CONTRIBUTING, PLAN) and in scripts/check-selectable-text.sh. It leaves docs/plans/ and .agent/memory/ as they are, since those are historical records.

Test plan

Local, Linux only:

• cargo build --workspace
• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace: 333 passed
• aura --version prints aura 0.1.36; aura doctor and aura keys describe work
• scripts/check-selectable-text.sh
• scripts/prepare-release.sh --dry-run --no-fetch

Not yet run:

• Launch the tray app (cargo run -p aura) and open the modal
• macOS and Windows builds. Could not cross-compile locally; CI covers them