docs: document macros as first-class custom commands

Covers top-level .name invocation, the new description/isolated macro
fields (with the non-isolation persistence, fail-fast, nested-macro, and
.exit caveats), workspace .coyote/macros/ + --no-workspace-macros,
enabled_macros scoping at global/role/agent/session levels, and
.macro enable|disable across the README and every example config.
graph.example.yaml gains a note that enabled_macros is ignored in graph
configs.

CHANGELOG intentionally untouched: it is generated by commitizen at
release time from the conventional commit subjects.

Implements plans/custom-commands-design.md §8.

graph.example.yaml

@@ -68,6 +68,9 @@ enabled_skills:
 inject_skill_instructions: true   # Inject a hint pointing the model at `skill__list`. Defaults to true; suppressed
                                   # automatically when no skills are available.
 skill_instructions: null          # Custom text for the skill hint (optional; uses the built-in default if omitted).
+# Note: `enabled_macros` is NOT a graph setting. Graph nodes never dispatch REPL
+# commands, so the field is silently ignored in graph configs; scope macros via
+# the global config, roles, agents, or sessions instead.
 
 conversation_starters:             # Suggested prompts surfaced in the UI
   - "Research the current state of WebAssembly outside the browser"