feat(cli): interactive splash menu and grouped help (#3)

[rosspeili]

Description

Adds an interactive CLI experience when aura is invoked with no subcommand: ASCII AURA splash, numbered menu (agents, sessions, run, home, help, version), and Skillware-style grouped help via aura --help. Existing subcommands (aura agent, aura run, aura export, etc.) are unchanged for scripting and CI.

Refactors the CLI into focused modules (commands, splash, interactive, help_ui, styles, help_text) and adds rich as a core dependency for tables and the gradient splash. Includes UTF-8-safe output handling for piped/legacy Windows terminals.

Closes #3 — extends the CLI integration tests from #2 with grouped help, interactive exit, and console script entry point checks.

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 #3)
• pytest passes (53 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 — version, agent CRUD, run, logs/export/compare/otel, grouped --help, interactive splash + 0 exit, console script entry point
• pytest tests/ -q — full suite (53 passed)
• Manual: bare aura shows splash and menu; aura --help shows grouped topics; aura agent list / aura version still scriptable
• Verified wheel entry point: aura = "aura.cli.main:main" in pyproject.toml