poc-memory: POC_MEMORY_DRY_RUN=1 for agent testing

All mutating commands (write, delete, rename, link-add, journal write,
used, wrong, not-useful, gap) check POC_MEMORY_DRY_RUN after argument
validation but before mutation. If set, process exits silently — agent
tool calls are visible in the LLM output so we can see what it tried
to do without applying changes.

Read commands (render, search, graph link, journal tail) work normally
in dry-run mode so agents can still explore the graph.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-memory/src/cli/graph.rs

@@ -134,6 +134,7 @@ pub fn cmd_triangle_close(min_degree: usize, sim_threshold: f32, max_per_hub: us
 }
 
 pub fn cmd_link_add(source: &str, target: &str, reason: &[String]) -> Result<(), String> {
+    super::check_dry_run();
     let mut store = store::Store::load()?;
     let source = store.resolve_key(source)?;
     let target = store.resolve_key(target)?;