feat: surface macros as first-class custom commands in the REPL

Implements the invocation and management surfaces from
plans/custom-commands-design.md §4 and §6:

- Top-level dispatch: an enabled macro <name> now runs as ".<name> [args]"
  from the command catch-all; runtime-disabled macros point at
  ".macro enable <name>", locked macros name the owning config, and
  unknown commands keep the existing error verbatim
- .macro enable|disable <name>: runtime toggles over the in-memory
  global-level enabled_macros list (disable with no list materializes
  all-active-minus-name); toggles error when a role/agent/session
  allowlist owns the field
- .set enabled_macros <csv|null> with workspace-then-global existence
  validation; .set key completion gains enabled_macros and the
  previously missing enabled_skills
- Dynamic completion: enabled macros (with descriptions) join built-ins
  on ".<TAB>" without touching the static command registry;
  ".macro <TAB>" lists invocable macros (incl. built-in-shadowed ones)
  plus the enable/disable subcommands; second-arg completion offers
  toggle-eligible names
- .list macros: enriched table (name, source, isolated, state,
  description) covering every resolver state incl. missing and
  shadowed rows; .help gains a custom-commands section
- Session info/render and sysinfo display enabled_macros; Macro::load
  resolves workspace-then-global; enable/disable rejected as macro
  names in the creator

src/config/mod.rs

@@ -31,10 +31,8 @@ pub use self::app_config::AppConfig;
 pub use self::app_state::AppState;
 pub use self::input::Input;
 pub use self::install_remote::{install_remote, install_remote_from_repl_args};
-#[allow(unused_imports)]
 pub use self::macro_policy::{
-    DiscoveredMacro, MacroAllowlistLevel, MacroPolicy, MacroSource, MacroState,
-    RESERVED_MACRO_NAMES, ResolvedMacro, discover_macros,
+    MacroAllowlistLevel, MacroPolicy, MacroSource, MacroState, RESERVED_MACRO_NAMES, ResolvedMacro,
 };
 #[allow(unused_imports)]
 pub use self::request_context::{RenderMode, RequestContext, should_inject_skill_instructions};