feat(cli): redesign context-stats with action-based interface (#50)

[luongnv89]

Closes #50

Summary

Implements a strict, explicit action-based CLI interface for context-stats. The design enforces a clean, predictable pattern: users must always specify both the session ID and the action, with no hidden defaults or convenience shortcuts.

Pattern

context-stats <session_id> <action> [parameters]

The only exception is the explain action, which reads JSON from stdin and doesn't require a real session file, so it can be called as:

echo '{}' | context-stats explain

Approach

• Strict argument parsing: both session_id and action are required positional arguments
• Session ID validation prevents path traversal and injection attacks
• Clean separation of action-specific logic
• Explain action as special case for piped input

Changes

File	Change
src/claude_statusline/cli/context_stats.py	Strict session-first action-based parsing
tests/python/test_export.py	Updated to use new required pattern
tests/python/test_state_rotation_validation.py	Updated to use new required pattern

Test Results

• All 317 Python tests pass ✓
• All 84 Node.js tests pass ✓
• Argument parsing validated for strict pattern
• Session ID validation working correctly

Supported Usage

# Show live graphs
context-stats abc123def graph

# Show graphs once and exit
context-stats abc123def graph --no-watch

# Show only cumulative graph
context-stats abc123def graph --type cumulative

# Export session stats
context-stats abc123def export --output report.md

# Diagnostic dump (special case - no session needed)
echo '{"model":...}' | context-stats explain

Acceptance Criteria

• Implement strict <session_id> <action> [parameters] pattern
• Support three actions: graph, export, explain
• Require both session ID and action for all operations
• Allow explain exception for stdin piping
• Validate session IDs to prevent injection
• All tests pass