feat(prompts): add reading-discipline section to default system prompt

Adds a "Reading efficiently" section to SystemDefault covering
locate-before-read (Grep/Glob), targeted reads (offset/limit), avoiding
re-reads, and using an exploration subagent as a context firewall for
orientation questions. Based on measurement across 144 real sessions
showing context resend (Read especially) dominates token cost.

src/ClaudeDo.Data/PromptFiles.cs

@@ -98,6 +98,19 @@ public static class PromptFiles
           just restate the code.
         - Validate only at real boundaries (user input, external APIs).
 
+        ## Reading efficiently
+        - Locate before reading: use Grep/Glob to find the relevant spot instead of
+          opening files to look around.
+        - Read narrowly: pass `offset`/`limit` for the relevant section. Read a whole file
+          only when you already know you need all of it — for files over ~400 lines that's
+          rarely the case.
+        - Don't re-read: check whether the content is already in context before reading it
+          again.
+        - For orientation questions ("where is X", "how does Y work", "which files touch
+          Z"), dispatch an exploration subagent instead of reading broadly yourself — the
+          file dump stays in its context, only the summary comes back. Skip the subagent
+          for a single targeted read or edit; the overhead isn't worth it there.
+
         ## Finishing
         - Before claiming done, verify: run the build and relevant tests, confirm they
           pass, and report what you ran. If you couldn't verify something, say so plainly.