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/session.rs

@@ -113,6 +113,10 @@ impl Session {
         self.enabled_skills.as_deref()
     }
 
+    pub fn enabled_macros(&self) -> Option<&[String]> {
+        self.enabled_macros.as_deref()
+    }
+
     pub fn set_skills_enabled(&mut self, value: Option<bool>) {
         if self.skills_enabled != value {
             self.skills_enabled = value;