feat: add lazy macro resolver with two-dir discovery and per-macro states

Adds src/config/macro_policy.rs: MacroPolicy::effective computes the
visible macro set on demand from the discovered definition files, the
four-level enabled_macros allowlists, and the built-in command names.

- Discovery scans workspace (.coyote/macros/) then global macros dirs on
  every resolution; workspace shadows global by name, and the shadowed
  global entry is retained and flagged so both stay listable (plan
  custom-commands-design.md §5). Workspace scanning is gated on a bool
  parameter so the future --no-workspace-macros flag wires in one line.
- Allowlist precedence is session > agent > role > global, first Some
  wins, no merging; None falls through, an empty list is an explicit
  zero, all-None enables everything (mirrors SkillPolicy).
- Per-macro states per plan §6: enabled, disabled (runtime, global-level
  exclusions only), locked (role/agent/session exclusions, recording the
  owning level), missing (unknown allowlist names warn instead of
  bailing — deliberate divergence from skills), shadowed (built-in name
  collisions), and invalid (parse failures and the reserved names
  enable/disable). Invalid beats allowlist exclusion beats shadowing.
- Adds enabled_macros() accessors on Role, Session, and Agent alongside
  their enabled_skills() counterparts, plus paths::workspace_macros_dir.
- 37 tests: state matrix, pairwise precedence, explicit-zero pinned at
  every level, workspace shadowing, reserved names, builtin collisions,
  missing rows, invalid YAML, and env-gated discovery (#[serial]).

src/config/mod.rs

@@ -4,6 +4,7 @@ mod app_state;
 mod input;
 mod install_remote;
 pub(crate) mod instructions;
+mod macro_policy;
 mod macros;
 mod mcp_factory;
 pub(crate) mod memory;
@@ -31,6 +32,11 @@ 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,
+};
+#[allow(unused_imports)]
 pub use self::request_context::{RenderMode, RequestContext, should_inject_skill_instructions};
 pub use self::role::{
     CODE_ROLE, CREATE_TITLE_ROLE, EXPLAIN_SHELL_ROLE, Role, RoleLike, SHELL_ROLE,