fix(security): index config files by key only, never surface values (#383) (#722)

Spring `application.{properties,yml}` keys (and Shopify Liquid `{% schema %}`
blocks) were storing the config VALUE in the node docstring, and
`codegraph_explore`'s source section re-read the raw `key = value` line off
disk — so a secret committed to a config file (DB password, API key, JDBC URL
with embedded credentials) could be pushed into an agent's context via
explore/node output without the agent ever opening the file.

Config-leaf nodes (`kind: 'constant'` in a config language) now surface the KEY
only, via a shared `isConfigLeafNode` predicate applied at both surfacing
paths: the value is dropped from extraction, `getCode`/`includeCode` returns
the key instead of the file line, and explore excludes config leaves from
source rendering. The predicate can't match real code (real constants are
ts/java/go/…), so `@Value`/`@ConfigurationProperties` resolution and impact are
unaffected. Adds a regression test asserting a planted secret never appears in
`codegraph_explore` / `codegraph_node` output while the keys still resolve.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/extraction/liquid-extractor.ts

@@ -313,7 +313,10 @@ export class LiquidExtractor {
         endLine,
         startColumn: match.index - this.getLineStart(startLine),
         endColumn: 0,
-        docstring: schemaContent?.trim().substring(0, 200), // Store first 200 chars as docstring
+        // SECURITY (#383): don't dump the raw {% schema %} JSON (section settings
+        // + default values) into the docstring — the schema name is already in
+        // `name`, so the data block adds nothing but a potential leak of any
+        // IDs/endpoints/keys a developer placed in setting defaults.
         updatedAt: Date.now(),
       };