## Exploration: issue #542 session command registry migration

### Current State
`/resume`, `/suspend`, `/tldr`, and `/compact` already execute through the central registry in production code. `pre_execution::evaluate_ingress(...)` constructs `SessionCommandService`, calls `default_registry().dispatch(...)`, and returns early before any blocking fallback. The CLI fast path (`main.rs`), gateway HTTP early response and SSE path (`gateway/mod.rs`), webhook dispatcher (`gateway/webhook_dispatch.rs`), and channel ingress (`channels/mod.rs`) all route recognized slash input through that shared seam rather than calling session-command handlers directly.

There is no remaining production path that invokes `SessionCommandService::handle_resume`, `handle_suspend`, `handle_tldr`, or `handle_compact` directly outside the registry handlers in `session_commands/registry.rs`. The remaining legacy surface is routing/adaptation glue, not duplicate command execution.

### Affected Areas
- `clients/agent-runtime/src/pre_execution/mod.rs` — canonical ingress seam; registry dispatch already happens here.
- `clients/agent-runtime/src/session_commands/registry.rs` — built-in registrations and the only production bindings from command names to handlers.
- `clients/agent-runtime/src/session_commands/service.rs` — preserved command behavior and authz/backend checks behind registry handlers.
- `clients/agent-runtime/src/main.rs` — CLI compatibility shim still named around “session commands,” but already calls the shared ingress seam.
- `clients/agent-runtime/src/gateway/mod.rs` — legacy HTTP/SSE transport still keeps an early-response helper for deterministic slash interception before plan/cost guards.
- `clients/agent-runtime/src/gateway/webhook_dispatch.rs` — dispatcher path already uses the shared ingress seam.
- `clients/agent-runtime/src/channels/mod.rs` — channel ingress already uses the shared ingress seam.

### Approaches
1. **Declare #542 complete as-is** — treat the current state as satisfying the migration.
   - Pros: no code churn; execution is already registry-backed.
   - Cons: leaves small cleanup debt and “session command” compatibility naming that makes completion less obvious during review.
   - Effort: Low

2. **Do a closure cleanup slice** — keep behavior unchanged, but remove the leftover migration noise.
   - Pros: smallest code change that makes completion obvious; avoids broadening into new command families.
   - Cons: mostly cleanup/documentation/tests rather than functional migration.
   - Effort: Low

### Recommendation
Use **Approach 2**. The functional migration is already done, so the smallest focused slice is a closure pass that removes or isolates the last compatibility artifacts: drop the unused `SlashCommandRegistry::recognizes(...)` helper, rename transport-local helpers/comments that still imply a separate session-command path, and add/refresh targeted tests that assert all four commands short-circuit through `pre_execution::evaluate_ingress(...)` across the existing ingress surfaces. That closes #542 cleanly without inventing new slash command families.

### Risks
- Reviewers may expect visible behavior changes even though the remaining work is mostly cleanup and proof.
- Renaming helper functions/comments can create small merge conflicts with nearby slash-command work such as #543.
- If the team interprets “old special-case routing” to include transport-specific outward envelope adaptation, #542 could be blocked by a larger refactor; current spec language does not require that refactor.

### Ready for Proposal
Yes — propose a narrow cleanup/proof change named `finalize-session-command-registry-routing` focused on isolating/removing leftover migration shims without changing command behavior.
