Install unqualified agent commands as wrappers

[robobryce]

Summary

• Install the unqualified claude and codex commands as regular AAB launcher files instead of provider symlinks. The selected claude entrypoint is written as a launcher file at ~/.local/aab-bin/claude (kept ahead of ~/.local/bin on PATH, where the native auto-updater owns ~/.local/bin/claude); codex is written at ~/.local/bin/codex.
• Keep explicit provider launchers available, such as claude-third-party-deepseek and codex-third-party-openai.
• Keep ~/.bashrc alias-free; it only manages PATH and non-secret unattended-mode exports.
• Update tests and README docs for the regular-file launcher behavior.

Rebased onto upstream/main, which had rearchitected the launcher (native binary stays at ~/.local/bin/claude, AAB entrypoint moved to ~/.local/aab-bin, plus a new third-party-nemotron provider). This change now installs that entrypoint as a launcher file rather than a symlink to a provider wrapper, and the nemotron unit test was updated to match.

Tests

• ./test.bash (lint + unit) — lint clean; unit 1..108, all ok, 0 failures.
• ./test.bash --docker — full e2e in fresh ubuntu:22.04: All e2e assertions passed. / === docker e2e passed ===. Covers claude wrapper family installed and selected (launcher dir wins on PATH) and codex wrapper family installed and runnable.
• ./test.bash --secrets — gitleaks: no leaks found.
• git diff --check upstream/main..HEAD — no whitespace/conflict errors; branch is 1 commit ahead, 0 behind upstream/main.