fix(hooks-routing): register model_role_resolver capability in mount()

[bkrabach]

What

Registers the model_role_resolver capability in hooks-routing mount() so that consumers calling coordinator.get_capability("model_role_resolver") actually receive the matrix-strategy resolver.

Why

The MatrixModelRoleResolver class shipped in resolver_class.py (introduced in commit 3f20ad3) was never wired into the capability registry. As a result:

1. Every delegate(..., model_role=X, ...) call where the caller supplied a model_role triggered the stderr warning at amplifier-foundation/modules/tool-delegate/__init__.py:790-794:

   model_role 'X' specified but no model_role_resolver capability is registered (install a routing bundle)
   

   despite this bundle being mounted and the matrix loaded correctly.

2. Silent functional regression: the caller's model_role override was discarded at tool-delegate/__init__.py:781-806. The spawn proceeded with the agent's pre-resolved default provider_preferences instead, ignoring caller intent.

Evidence

Reproduced end-to-end in a Digital Twin Universe environment running upstream main:

• Before fix: amplifier tool invoke delegate agent=foundation:explorer model_role=writing instruction="..." emitted the warning above on every call (deterministic — confirmed with both writing and coding roles, byte-identical event sequences across runs, ruling out a race condition).
• After fix: warning is gone. The replacement warning, when triggered by empty installed-provider matches, is model_role 'X' resolved to no candidates against installed providers (resolver=balanced) — emitted from the success branch of the capability check (tool-delegate/__init__.py:801-806), proving the resolver is now registered, looked up, and invoked. The matrix name balanced is correctly reported via MatrixModelRoleResolver.name.

Changes

• modules/hooks-routing/amplifier_module_hooks_routing/__init__.py (+17 lines): instantiate MatrixModelRoleResolver after effective_matrix is composed, register via coordinator.register_capability("model_role_resolver", resolver). Guarded by effective_matrix truthy and hasattr(coordinator, "register_capability").
• modules/hooks-routing/tests/test_init.py (+84 lines): new TestModelRoleResolverCapability test class asserting register_capability("model_role_resolver", ...) is called from mount() and the registered resolver's resolve("fast") returns a non-empty list of ProviderPreference.

All 8 tests in modules/hooks-routing/tests/test_init.py pass.

Cascade and fallback semantics

The fix preserves the existing five-step fallback cascade. Confirmed by code reading:

1. Caller-supplied provider_preferences (delegate input) — highest priority
2. Caller-supplied model_role — now actually honored (was previously discarded)
3. Agent's pre-resolved provider_preferences (from session:start hook, resolved from agent's frontmatter model_role chain)
4. Agent's frontmatter provider_preferences: (hard-pinned, bundle-portable)
5. provider_preferences=None → child spawn inherits parent's current provider

The role-level fallback chain (resolve_model_role iterating an ordered list) is unchanged. Agent-frontmatter model_role: [list] still honors chains.

Behavioral change to flag

Any caller that was incidentally relying on the override being silently discarded (i.e., expecting the agent's default model_role chain to win even when passing model_role=X to delegate) will see different behavior after this fix. I have no evidence such callers exist in the wild, but the bug-fix-shifts-behavior risk is non-zero — worth a heads-up.

Out of scope (intentionally)

• The delegate-tool input schema accepts model_role as a string only, not a list. The resolver itself and the agent-frontmatter path both accept list-form chains. This asymmetry pre-dates the bug and is not addressed here.
• Version bump in bundle.md: deferred. The matrix bundle has no established versioning discipline for hook-only behavior fixes. Maintainer call.

Verification commands

# In-tree unit tests
pytest modules/hooks-routing/tests/ -v

# End-to-end (any DTU or local install with at least one provider installed)
amplifier tool invoke delegate agent=foundation:explorer model_role=fast instruction="Say hello"
# Expected stderr: empty for "no model_role_resolver capability is registered"