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>

CHANGELOG.md

@@ -9,6 +9,10 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Security
+
+- CodeGraph now indexes Spring configuration files (`application.properties` / `application.yml`) by key only, and never includes their values in `codegraph_explore` or `codegraph_node` output. Previously a secret committed to one of these files — a database password, API key, or connection string with embedded credentials — could be surfaced to an AI agent that asked about nearby code, even though the agent never opened the file. The configuration keys are still indexed, so reference and impact analysis are unaffected; an agent that genuinely needs a value reads the file itself. Shopify Liquid `{% schema %}` blocks are likewise indexed by name only. (#383)
+
 ### New Features
 
 - New `codegraph upgrade` command updates CodeGraph to the latest release in place — it detects how you installed (the standalone `install.sh` / `install.ps1` bundle, npm, or npx) and does the right thing for each, on macOS, Linux, and Windows. Use `codegraph upgrade --check` to see whether an update is available without installing, or `codegraph upgrade <version>` to move to a specific version. After upgrading it reminds you to re-index your projects so they pick up the newer engine's improvements. (#679)