feat(agent): .hermes.md per-repository project config discovery

Adds .hermes.md / HERMES.md discovery for per-project agent configuration.
When the agent starts, it walks from cwd to the git root looking for
.hermes.md (preferred) or HERMES.md, strips any YAML frontmatter, and
injects the markdown body into the system prompt as project context.

- Nearest-first discovery (subdirectory configs shadow parent)
- Stops at git root boundary (no leaking into parent repos)
- YAML frontmatter stripped (structured config deferred to Phase 2)
- Same injection scanning and 20K truncation as other context files
- 22 comprehensive tests

Original implementation by ch3ronsa. Cherry-picked and adapted for current main.

Closes #681 (Phase 1)

agent/prompt_builder.py

@@ -56,6 +56,61 @@ def _scan_context_content(content: str, filename: str) -> str:
 
     return content
 
+
+def _find_git_root(start: Path) -> Optional[Path]:
+    """Walk *start* and its parents looking for a ``.git`` directory.
+
+    Returns the directory containing ``.git``, or ``None`` if we hit the
+    filesystem root without finding one.
+    """
+    current = start.resolve()
+    for parent in [current, *current.parents]:
+        if (parent / ".git").exists():
+            return parent
+    return None
+
+
+_HERMES_MD_NAMES = (".hermes.md", "HERMES.md")
+
+
+def _find_hermes_md(cwd: Path) -> Optional[Path]:
+    """Discover the nearest ``.hermes.md`` or ``HERMES.md``.
+
+    Search order: *cwd* first, then each parent directory up to (and
+    including) the git repository root.  Returns the first match, or
+    ``None`` if nothing is found.
+    """
+    stop_at = _find_git_root(cwd)
+    current = cwd.resolve()
+
+    for directory in [current, *current.parents]:
+        for name in _HERMES_MD_NAMES:
+            candidate = directory / name
+            if candidate.is_file():
+                return candidate
+        # Stop walking at the git root (or filesystem root).
+        if stop_at and directory == stop_at:
+            break
+    return None
+
+
+def _strip_yaml_frontmatter(content: str) -> str:
+    """Remove optional YAML frontmatter (``---`` delimited) from *content*.
+
+    The frontmatter may contain structured config (model overrides, tool
+    settings) that will be handled separately in a future PR.  For now we
+    strip it so only the human-readable markdown body is injected into the
+    system prompt.
+    """
+    if content.startswith("---"):
+        end = content.find("\n---", 3)
+        if end != -1:
+            # Skip past the closing --- and any trailing newline
+            body = content[end + 4:].lstrip("\n")
+            return body if body else content
+    return content
+
+
 # =========================================================================
 # Constants
 # =========================================================================
@@ -441,6 +496,28 @@ def build_context_files_prompt(cwd: Optional[str] = None) -> str:
         cursorrules_content = _truncate_content(cursorrules_content, ".cursorrules")
         sections.append(cursorrules_content)
 
+    # .hermes.md / HERMES.md — per-project agent config (walk to git root)
+    hermes_md_content = ""
+    hermes_md_path = _find_hermes_md(cwd_path)
+    if hermes_md_path:
+        try:
+            content = hermes_md_path.read_text(encoding="utf-8").strip()
+            if content:
+                content = _strip_yaml_frontmatter(content)
+                rel = hermes_md_path.name
+                try:
+                    rel = str(hermes_md_path.relative_to(cwd_path))
+                except ValueError:
+                    pass
+                content = _scan_context_content(content, rel)
+                hermes_md_content = f"## {rel}\n\n{content}"
+        except Exception as e:
+            logger.debug("Could not read %s: %s", hermes_md_path, e)
+
+    if hermes_md_content:
+        hermes_md_content = _truncate_content(hermes_md_content, ".hermes.md")
+        sections.append(hermes_md_content)
+
     # SOUL.md from HERMES_HOME only
     try:
         from hermes_cli.config import ensure_hermes_home