fix(lint): rephrase too-large composition warnings to give actionable reasoning

Both `composition_file_too_large` and `timeline_track_too_dense` previously
said "Agents produce better results when large scenes are split into smaller
sub-compositions." The audience-flavored framing ("Agents produce better
results") doesn't tell a reader (agent or human) WHY smaller is better.

Reframe to concrete properties of smaller compositions: easier to read,
iterate on, and diff. The fixHint already covers the inspect/revise/validate
detail; the message now leads with a tight reason.

Per Abhay in #C0ACCNHLG3U:
> "an agent reading 'Agents produce better results' sounds weird. We should
>  give the agent an actual reason why smaller is better for them."

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/lint/rules/composition.ts

@@ -49,7 +49,7 @@ export const compositionRules: Array<(ctx: LintContext) => HyperframeLintFinding
       {
         code: "composition_file_too_large",
         severity: "warning",
-        message: `This HTML composition file has ${lineCount} lines. Agents produce better results when large scenes are split into smaller sub-compositions.`,
+        message: `This HTML composition file has ${lineCount} lines. Smaller sub-compositions are easier to read, iterate on, and diff.`,
         fixHint: `${splitTarget}, then mount them from the parent with data-composition-src so each file stays small enough to inspect, revise, and validate independently.`,
       },
     ];
@@ -77,7 +77,7 @@ export const compositionRules: Array<(ctx: LintContext) => HyperframeLintFinding
       findings.push({
         code: "timeline_track_too_dense",
         severity: "warning",
-        message: `Track ${track} has ${count} timed elements in this HTML file. Agents produce better timelines when dense tracks are split into smaller sub-compositions.`,
+        message: `Track ${track} has ${count} timed elements in this HTML file. Smaller sub-compositions keep timelines easier to read, iterate on, and diff.`,
         fixHint: `${splitTarget} and mount them from the parent with data-composition-src so the timeline stays easier to inspect, revise, and validate.`,
       });
     }