P5: centralize agent type aliases into single source of truth

[rsnodgrass]

Parent Epic

Part of #396 (unify agent abstraction layer)

Problem

Agent type alias resolution exists in 3 separate places:

1. internal/prime/agent_type.go:CanonicalAgentType() — maps "claude"→"claude-code", "gemini-cli"→"gemini", etc.
2. internal/session/adapters/*.go:init() — each adapter registers with aliases
3. cmd/ox/agent_hook.go:resolveAgentType() — another alias resolution for hook context

These can drift out of sync. When adding a new agent, all 3 must be updated.

Proposed Solution

Define aliases in agent metadata (from P1). Then:

• CanonicalAgentType() iterates registry metadata to find matching alias
• Adapter Register() calls use aliases from metadata
• resolveAgentType() delegates to CanonicalAgentType()

Single source of truth: Agent.Metadata().Aliases.

Dependency

Depends on P1 (Agent.Metadata() interface).

Files to Change

• internal/prime/agent_type.go — simplify using registry aliases
• cmd/ox/agent_hook.go — delete resolveAgentType(), use canonical function
• internal/session/adapters/*.go — derive aliases from metadata (or keep init() but validate against metadata)

Risk

• Blast radius: Small (2-3 files)
• Behavior change: None — alias resolution is centralized