fix(pg): align SearchByEmbedding SQL placeholders with scope args

[wwenrr]

Summary

Fix placeholder index alignment in PGSkillStore.SearchByEmbedding when scope args are present/absent.

Problem

Issue #601 reports:
SQLSTATE 42P18: could not determine data type of parameter $2

SearchByEmbedding built a dynamic query with hardcoded placeholder offsets (starting scope at $3) while argument composition was dynamic. This could produce placeholder/index mismatch depending on scope shape.

Changes

• internal/store/pg/skills_embedding.go
  • start scope placeholders at $2 (after $1 vector)
  • use nextParam returned by scopeClause() for ORDER BY ... <=> $n
  • use n+1 for LIMIT
  • centralize tenant/system condition formatting in helper:
    • buildSkillEmbeddingTenantCond(scope string)
• internal/store/pg/skills_embedding_test.go
  • add tests for tenant condition builder:
    • empty scope
    • tenant-only scope
    • tenant + project scope

Closes #601