gittensor-vanguard/vanguarstew: add label multipliers and scoring config

[matedev01]

Adds per-repo configuration for gittensor-vanguard/vanguarstew in master_repositories.json.

• label_multipliers: a maintainer-applied value ladder — mult:core-correctness (2.0), mult:leakage-integrity (1.8), mult:capability (1.5), mult:enhancement (1.2), mult:maintenance (1.0), mult:docs (0.8).
• default_label_multiplier: 1.0 (neutral for untriaged PRs).
• trusted_label_pipeline: false — only maintainer-associated labels count toward the multiplier.
• scoring.pr_lookback_days: 30.

emission_share (0.01) and issue_discovery_share (0.0) are unchanged; no other repo entries are touched. Total emission_share still sums to 1.0. Values are within the documented ranges (pr_lookback_days ∈ [1, 90]).

[anderdc]

PRs need to target test branch