feat(prompt): make context-file truncation limit configurable

PROBLEM: Automatic context files such as SOUL.md and AGENTS.md were capped by a hardcoded CONTEXT_FILE_MAX_CHARS value. Amy's local fork had raised that constant from 20K to 25K so a larger SOUL.md would not be silently truncated, but the hardcoded 25K value changed upstream default behavior and made the patch less generally useful.

SOLUTION: Restore the upstream-compatible 20K default, add a context_file_max_chars config setting for users who intentionally keep larger identity/project-context files, keep chat-visible truncation warnings, and document the new setting. Tests cover the default, config override, explicit max_chars precedence, and the warning text.

hermes_cli/config.py

@@ -1104,6 +1104,11 @@ DEFAULT_CONFIG = {
         "min_interval_hours": 24,
     },
 
+    # Maximum characters loaded from a single automatic context file such as
+    # SOUL.md, AGENTS.md, CLAUDE.md, .hermes.md, or .cursorrules before Hermes
+    # applies head/tail truncation. This is separate from read_file tool limits.
+    "context_file_max_chars": 20_000,
+
     # Maximum characters returned by a single read_file call.  Reads that
     # exceed this are rejected with guidance to use offset+limit.
     # 100K chars ≈ 25–35K tokens across typical tokenisers.