feat(matplotlib): implement line-basic (#476)

## Summary
Implements `line-basic` for **matplotlib** library.

**Parent Issue:** #205
**Sub-Issue:** #452
**Base Branch:** `plot/line-basic`
**Attempt:** 1/3

## Implementation
- `plots/matplotlib/plot/line-basic/default.py`

## Changes
- Basic line plot implementation using matplotlib's `ax.plot()` method
- Uses Python Blue color (#306998) from the style guide
- Includes markers (8px) for data point visibility
- Follows spec data structure with time/value columns
- Proper axis labels, title, and grid styling

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>

Author: claude[bot]

plots/matplotlib/plot/line-basic/default.py

@@ -12,7 +12,7 @@
 
 # Create plot
 fig, ax = plt.subplots(figsize=(16, 9))
-ax.plot(data["time"], data["value"], linewidth=2, color="#306998", marker="o", markersize=4)
+ax.plot(data["time"], data["value"], linewidth=2, color="#306998", marker="o", markersize=8)
 
 # Labels and styling
 ax.set_xlabel("Time", fontsize=20)