fix(repo-stats): plot ghcr pulls on a linear axis so the trend is visible

The ghcr pulls chart used a symlog y axis with a hardcoded tick list, both
copied from github-repo-stats, which builds the rest of the report. Those
settings suit views and clones - small, spiky, frequently zero - but not
container pulls, which sit in a tight band far above zero.

Consequences on the published report: the series (7,946 to 16,160) lives
entirely inside the top decade of the log scale, so a 2x swing rendered as a
14px wobble on a 200px chart and read as a flat line. Of the nine fixed ticks,
six were squashed against the baseline and 50000 fell outside the domain and
never drew, leaving a single usable gridline.

Switch to a linear scale, drop the fixed ticks so Vega derives them from the
actual domain, and format labels with SI prefixes (5k / 10k / 15k). The zero
baseline and the 10% headroom are unchanged, so the axis stays honest; the same
swing now spans 92px and the growth from ~9k to ~15k pulls/day is legible.

.github/scripts/ghcr_inject.py

@@ -181,11 +181,13 @@ def build_vega_spec(series: dict[str, int]) -> dict:
                 "type": "temporal",
             },
             "y": {
-                "axis": {"values": [1, 10, 50, 100, 500, 1000, 5000, 10000, 50000]},
+                # Linear, not symlog: pulls sit in a tight band far above zero, so a log-ish
+                # axis squeezes the whole series into its top decade and flattens the line.
+                "axis": {"format": "~s"},
                 "field": "daily_count",
                 "scale": {
                     "domain": [0, y_max * 1.1 if y_max > 0 else 1],
-                    "type": "symlog",
+                    "type": "linear",
                     "zero": True,
                 },
                 "title": "container pulls per day",