feat(cli): agent set, config/paths, splash polish (Fixes #8) - #9

Description

Closes the post–#3 CLI UX gap: users can now edit agent profiles and persist storage paths without hand-editing JSON. Adds aura agent set, aura config show, and aura paths (set-project, set-storage), plus interactive paths and agents → edit menus. Splash gets a leading blank line and smoother Rich truecolor gradient on Windows Terminal.

Uses existing storage model (AgentRegistry.save(), ~/.aura/config.yaml, aura.project.yaml) — no brain bindings or secrets vault. Optional project_dir in global config is loaded on startup.

Fixes #8.

Type

• Bug fix
• Core framework — session, spine, constraints, identity, audit report
• CLI
• Integration — sequencer, hosts, membrane, observers, exporters
• Documentation
• Examples
• Packaging / CI

Checklist

• Linked issue (Fixes #8)
• pytest passes (55 passed)
• black aura tests — no diff
• flake8 aura tests — clean
• CHANGELOG.md updated when user-visible behavior changes

Test plan

• pytest tests/test_cli.py -v — agent set, config show, paths set-project/set-storage, splash leading newline + exit
• pytest tests/ -q — full suite (55 passed)
• Manual: aura agent set agent_55 --ref rosspeili/agent_55 --variable model=llama3.2 → aura agent show agent_55
• Manual: aura config show, aura paths, aura paths set-project ., interactive menu 4 paths and 1 agents → 4 edit
• Manual: bare aura — blank line above ASCII logo, gradient on Windows Terminal