Add --profile flag for per-account state isolation

[guimou]

Summary

When using ccbox with multiple accounts (e.g. one on Vertex AI, one on Anthropic), a single shared ~/.claude directory blends credentials, session history, and /usage statistics across accounts — /usage is computed locally from session transcripts and the stats cache, so both accounts accumulate into one pool.

This PR adds a --profile <name> flag that keeps each account fully separate:

• Isolated per profile (under ~/.claude-profiles/<name>/): .credentials.json, .claude.json, statsig/, stats-cache.json, caches (file-history, paste-cache, cache, backups), and per-project data (ccbox-projects/ — session transcripts, history, todos, plans, tasks, plugins). This is everything /usage reads, so each profile gets independent usage statistics.
• Shared across profiles (still from ~/.claude): settings, keybindings, CLAUDE.md, hooks, commands, skills, agents, rules, themes, workflows — no duplication of customizations, no symlinks needed since ccbox mounts item-by-item.

CLAUDE_CODE_USE_VERTEX=1 ANTHROPIC_VERTEX_PROJECT_ID="my-project" ccbox --profile vertex
ccbox --profile anthropic
ccbox   # no flag = unchanged default behavior, shared ~/.claude

Details

• Container names become ccbox-{project}-{hash}-{profile}-{session-id}, so --list-sessions (unchanged, prefix-based) shows which profile each session belongs to.
• Profile names are validated: lowercase letters, digits, dashes, underscores.
• A new profile starts unauthenticated (run /login or use provider env vars) and re-runs first-time onboarding once, since its .claude.json starts empty.
• ensure_global_config is split into shared vs account-specific (ensure_account_config) halves; without --profile both run against ~/.claude, producing identical behavior to before.
• Docs updated: README (new "Account Profiles" section, architecture diagram, "Where Data Lives" table) and CLAUDE.md.

Testing

• ShellCheck: no new findings (the two pre-existing info/warning findings on main remain).
• Smoke tests with a stubbed podman and isolated $HOME:
  • --profile vertex: profile tree created, all account-specific mounts sourced from ~/.claude-profiles/vertex/, container name includes the profile.
  • No flag: generated podman run mounts identical to current behavior.
  • Invalid names (--profile "Bad Name") rejected with a clear error.
  • --list-sessions still works and matches profile-suffixed containers.