Reusable interceptors for system operations.

- timing:      Measures wall-clock duration, injects into effects
- journal:     Records operation to IJournal after execution
- tool-check:  Validates binary exists before shell exec
- result-lift: Converts raw handler return into Result-typed effect

All interceptors follow the re-frame context model:
{:coeffects {:event [...] :db ...} :effects {...}}

Event-driven system operations.

Registers event handlers for shell execution, tool checking,
and process lifecycle. Each handler returns a Result in the
effects map, which the journal interceptor persists.

Init:
  (init!)                        ;; default — no journal
  (init! {:journal-fn record!})  ;; with IJournal callback

Dispatch:
  (dispatch [:sys/exec "ls -la"])
  (dispatch [:sys/exec ["rg" "pattern" "src/"]])
  (dispatch [:sys/exec-ok "make test"])
  (dispatch [:sys/which "rg"])
  (dispatch [:sys/require-tool :ripgrep])
  (dispatch [:sys/require-tools [:ripgrep :fd :jq]])

All handlers return {:result (ok ...) or (err ...)} in effects.
The timing interceptor adds :_duration-ms.
The journal interceptor persists to IJournal if configured.