[Feature] Session management safeguards and UX improvements

## Problem

Several session management commands have UX gaps:

1. **`altimate session delete` has no confirmation** — permanently deletes session history immediately with no safety net
2. **`altimate session list` with no sessions gives no output** — silent empty result, user can't tell if the command worked
3. **`altimate export` without session ID hangs** — starts interactive selection that doesn't work in non-interactive mode
4. **`altimate db` with no subcommand gives no output** — no help text or available subcommands shown

## Desired Behavior

### Session Deletion Safety
- Require confirmation before permanent deletion: "Session 'SQL Optimization' (15 messages, 3 tool calls). Delete? [y/N]"
- Support `--force` flag to skip confirmation for scripts
- Consider soft-delete with configurable retention (e.g., 7 days) before permanent removal

### Empty State Handling
- `altimate session list` with no sessions → "No sessions found. Start one with: altimate"
- `altimate db` with no subcommand → show available subcommands and brief help
- `altimate export` without session ID in non-interactive mode → clear error with usage hint

## Key Files
- `packages/altimate-code/src/cli/cmd/session.ts` — session list, delete commands
- `packages/altimate-code/src/cli/cmd/export.ts` — export command
- `packages/altimate-code/src/cli/cmd/db.ts` — db command

## Industry Patterns
- **Claude Code**: Checkpointing system with `/rewind` — scrollable checkpoint list with options to rewind conversation only, code only, or both
- **Git**: `git branch -D` requires `-D` (force) for unmerged branches; regular `-d` has safety check
- **GitHub CLI**: All destructive operations require `--yes` flag or interactive confirmation

## Acceptance Criteria

- [ ] `session delete` requires confirmation prompt (or `--force` flag)
- [ ] Session summary shown before deletion (message count, date range)
- [ ] `session list` shows helpful message when no sessions exist
- [ ] `export` in non-interactive mode gives clear error when no session ID provided
- [ ] `db` with no subcommand shows help text