feat(agent): add configurable coding_instructions

agent.coding_instructions (a string or list) is appended to the coding brief as
its own stable system block, so users can pin project-wide workflow rules
without editing the shipped brief. Coding-posture only and cache-safe (resolved
once per session; takes effect next session). Empty by default.

agent/coding_context.py

@@ -353,6 +353,29 @@ def _coding_mode(config: Optional[dict[str, Any]]) -> str:
     return "auto"
 
 
+def _coding_instructions(config: Optional[dict[str, Any]]) -> str:
+    """Standing operator instructions for the coding posture (config).
+
+    ``agent.coding_instructions`` — a string or list of strings appended to the
+    coding brief as an extra stable system block, so a user can pin project-wide
+    coding-workflow rules (e.g. "for UI work don't run tsc/lint until I approve;
+    clean the diff before committing") without editing the shipped brief.
+    Cache-safe: resolved once per session into the stable system-prompt tier,
+    like the rest of the posture.
+    """
+    if config is None:
+        try:
+            from hermes_cli.config import load_config
+
+            config = load_config()
+        except Exception:
+            config = {}
+    raw = ((config or {}).get("agent", {}) or {}).get("coding_instructions", "")
+    if isinstance(raw, (list, tuple)):
+        return "\n".join(str(item).strip() for item in raw if str(item).strip())
+    return str(raw or "").strip()
+
+
 def _resolve_cwd(cwd: Optional[str | Path]) -> Path:
     if cwd:
         return Path(cwd).expanduser()
@@ -459,6 +482,9 @@ class RuntimeMode:
     # only to steer edit-format guidance toward the model's family — see
     # ``_edit_format_line``. Fixed for the session, so cache-safe.
     model: Optional[str] = None
+    # Standing operator instructions (``agent.coding_instructions``), appended
+    # as an extra stable system block. Empty unless the user configures it.
+    instructions: str = ""
 
     @property
     def kind(self) -> str:
@@ -505,6 +531,10 @@ class RuntimeMode:
         workspace = build_coding_workspace_block(self.cwd)
         if workspace:
             blocks.append(workspace)
+        # Operator instructions ride their own block so the brief (block 0) stays
+        # byte-stable and cache-keyed independently of user config.
+        if self.instructions:
+            blocks.append(f"Operator instructions (from config):\n{self.instructions}")
         return blocks
 
     def compact_skill_categories(self) -> frozenset[str]:
@@ -557,6 +587,7 @@ def resolve_runtime_mode(
         cwd=resolved_cwd,
         config_mode=mode,
         model=model,
+        instructions=_coding_instructions(config),
     )