Concept relationship layer: wikilink edges in rolegraph

Start with MarkdownDirectivesParseResult and build_thesaurus_from_haystack(), then inspect RoleGraph::new() and the affected terraphim_rolegraph, terraphim_automata, terraphim_types, and terraphim_service crates. Confirm how related_concepts from #604 should become ConceptEdge values, then verify that graph traversal, autocomplete, and configurable search ranking support the new relationships.

Summary

Add concept-to-concept edges to the rolegraph, derived from [[wikilinks]] extracted by the Obsidian parser (#604). Currently the thesaurus is flat (synonym -> concept). Wikilinks enable a second layer: concept -> related concept.

Current State

Thesaurus (flat):
  "context management" -> concept("context engineering")
  "semantic scope"     -> concept("context engineering")

Proposed State

Thesaurus (flat, unchanged):
  "context management" -> concept("context engineering")

Relationships (new):
  concept("context engineering") --Related--> concept("knowledge graph")
  concept("context engineering") --Related--> concept("ontology")
  concept("knowledge graph")     --ParentChild--> concept("context engineering")  // from folder hierarchy

Edge Types

pub enum EdgeType {
    Related,      // from [[wikilinks]]
    ParentChild,  // from folder hierarchy (subfolder = child)
    SeeAlso,      // future: explicit "see also::" directive
}

pub struct ConceptEdge {
    pub from: NormalizedTermValue,
    pub to: NormalizedTermValue,
    pub edge_type: EdgeType,
    pub weight: f64,  // default 1.0, can be boosted by frequency
}

Integration with RoleGraph

terraphim_rolegraph already supports node/edge relationships. Feed ConceptEdge data into the existing graph structure so that:

1. Search ranking: when searching for "context engineering", results mentioning related concepts ("knowledge graph", "ontology") get a relevance boost
2. Graph traversal: terraphim_agent ASCII graph visualization shows concept neighborhoods
3. Autocomplete: typing "context" could suggest both synonyms AND related concepts

Implementation

1. Extend MarkdownDirectivesParseResult to include Vec<ConceptEdge> (#604 provides related_concepts)
2. In build_thesaurus_from_haystack(), after building the flat thesaurus, construct edges from related_concepts
3. Feed edges into RoleGraph::new() alongside the thesaurus
4. Boost search results that match related concepts (configurable weight, default 0.3x of direct match)

Affected Crates

• terraphim_rolegraph (primary -- accept and store concept edges)
• terraphim_automata (pass edges from parser output)
• terraphim_types (ConceptEdge, EdgeType types)
• terraphim_service (use edges for search ranking boost)

Dependencies

• #604 Obsidian format parser (provides related_concepts extraction)

Estimated Effort

~1 day

Part of

Epic #603