fix: add source titles to MCP graph facts

[w3lld1]

What

• add source_title to MCP graph_explore and graph_impact fact payloads
• derive titles through the shared search.CitationTitleForResult helper
• batch-load each memory's source file and section once for the export
• add MCP regression coverage for the empty-section filename fallback

Why

MCP graph consumers currently receive only the raw source path, while CLI graph JSON exports also expose a human-readable source title. This brings the two graph surfaces into parity without duplicating citation-title logic.

Fixes #381

How

The existing per-export memory lookup now selects both source_file and source_section into a map keyed by memory ID. Each graph fact uses those cached fields with CitationTitleForResult, preserving the existing source filter and avoiding per-fact database queries.

Touched surfaces:

• internal/mcp/graph_tools.go
• internal/mcp/graph_tools_test.go

Product behavior changed: yes, additively; MCP graph fact JSON may now include source_title.
Benchmark/eval interpretation changed: no.

Testing

• go test ./internal/mcp -run 'TestMCPGraphExplore(SourceTitleFallsBackToBaseFilename|BySubject|SourceFilter)' -count=1 -v
• go test ./internal/mcp -count=1
• go test ./... -count=1
• go vet ./...
• go build ./cmd/cortex/
• python3 scripts/validate_visualizer_contract.py
• scripts/connectivity_smoke.sh
• git diff --check

Risk

Low. The field is additive and omitted when no title can be derived. Existing source filtering and result ordering are unchanged.

AI-assisted contribution note

The implementation and test were AI-assisted. I reviewed the issue requirements, repository contribution rules, source-title helper reuse, query batching, generated diff, commit identity, and all test output listed above.

Checklist

• Unit tests added/updated
• Code follows Go conventions (gofmt, go vet)
• No unrelated changes included
• Documentation update is not required for this additive payload parity fix

[hurttlocker]

Verified locally: clean build, full internal/mcp suite green. Extending loadMemorySourcesForNodeViews to carry the section in the same batched IN-query was the right instinct — you closed #381 without inheriting the N+1 the CLI path has (#380 is still open if you want to bring the CLI side up to this standard). Same shared CitationTitleForResult path, regression mirrors the CLI test. Two-for-two today — merging.