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.

website/docs/user-guide/configuration.md

@@ -606,6 +606,20 @@ memory:
 
 With `memory.write_approval: true`, memory writes need your approval before they land: interactive CLI turns prompt inline; messaging sessions and the background self-improvement review stage the write for `/memory pending` → `/memory approve <id>` / `/memory reject <id>` review. Toggle at runtime with `/memory approval on|off`. See [Controlling memory writes](/user-guide/features/memory#controlling-memory-writes-write_approval).
 
+## Context File Truncation
+
+Controls how much content Hermes loads from each automatic context file before applying head/tail truncation. This applies to files injected into the system prompt such as `SOUL.md`, `.hermes.md`, `AGENTS.md`, `CLAUDE.md`, and `.cursorrules`. It does **not** affect the `read_file` tool.
+
+```yaml
+context_file_max_chars: 20000  # default
+```
+
+Raise it when you intentionally keep larger identity or project-context files and run models with enough context window to carry them:
+
+```yaml
+context_file_max_chars: 25000
+```
+
 ## File Read Safety
 
 Controls how much content a single `read_file` call can return. Reads that exceed the limit are rejected with an error telling the agent to use `offset` and `limit` for a smaller range. This prevents a single read of a minified JS bundle or large data file from flooding the context window.
@@ -1839,7 +1853,7 @@ Hermes uses two different context scopes:
 - **Project context files use a priority system** — only ONE type is loaded (first match wins): `.hermes.md` → `AGENTS.md` → `CLAUDE.md` → `.cursorrules`. SOUL.md is always loaded independently.
 - **AGENTS.md** is hierarchical: if subdirectories also have AGENTS.md, all are combined.
 - Hermes automatically seeds a default `SOUL.md` if one does not already exist.
-- All loaded context files are capped at 20,000 characters with smart truncation.
+- All loaded context files are capped at `context_file_max_chars` characters (default 20,000) with smart truncation.
 
 See also:
 - [Personality & SOUL.md](/user-guide/features/personality)