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/features/context-files.md

@@ -109,7 +109,7 @@ Context files are loaded by `build_context_files_prompt()` in `agent/prompt_buil
 1. **Scan working directory** — checks for `.hermes.md` → `AGENTS.md` → `CLAUDE.md` → `.cursorrules` (first match wins)
 2. **Content is read** — each file is read as UTF-8 text
 3. **Security scan** — content is checked for prompt injection patterns
-4. **Truncation** — files exceeding 20,000 characters are head/tail truncated (70% head, 20% tail, with a marker in the middle)
+4. **Truncation** — files exceeding `context_file_max_chars` characters (default 20,000) are head/tail truncated (70% head, 20% tail, with a marker in the middle)
 5. **Assembly** — all sections are combined under a `# Project Context` header
 6. **Injection** — the assembled content is added to the system prompt
 
@@ -171,12 +171,12 @@ This scanner protects against common injection patterns, but it's not a substitu
 
 | Limit | Value |
 |-------|-------|
-| Max chars per file | 20,000 (~7,000 tokens) |
+| Max chars per file | `context_file_max_chars` (default 20,000, ~7,000 tokens) |
 | Head truncation ratio | 70% |
 | Tail truncation ratio | 20% |
 | Truncation marker | 10% (shows char counts and suggests using file tools) |
 
-When a file exceeds 20,000 characters, the truncation message reads:
+When a file exceeds the configured limit, the truncation message reads:
 
 ```
 [...truncated AGENTS.md: kept 14000+4000 of 25000 chars. Use file tools to read the full file.]
@@ -185,7 +185,7 @@ When a file exceeds 20,000 characters, the truncation message reads:
 ## Tips for Effective Context Files
 
 :::tip Best practices for AGENTS.md
-1. **Keep it concise** — stay well under 20K chars; the agent reads it every turn
+1. **Keep it concise** — stay under your configured `context_file_max_chars`; the agent reads it every turn
 2. **Structure with headers** — use `##` sections for architecture, conventions, important notes
 3. **Include concrete examples** — show preferred code patterns, API shapes, naming conventions
 4. **Mention what NOT to do** — "never modify migration files directly"