Add random pattern selection (-r / --pattern random)

Pick a different movement pattern every time a sweep is triggered, so the
motion varies across the day instead of repeating one shape.

- strategies.ts: add the `random` sentinel, `SELECTABLE_PATTERN_NAMES`,
  `isSelectablePattern`, and `createRandomPicker`. `random` is deliberately
  NOT a registry entry: it has no path of its own, so `STRATEGIES` stays a
  total lookup and `PATTERN_NAMES` keeps listing only real generators. The
  picker is a closure over `last`, giving a uniform draw that never returns
  the same pattern twice in a row. Building CANONICAL_PATTERNS from the
  selectable list makes both validation boundaries accept `random` (and
  loose spellings) for free, and extends the normalization-collision
  assertion to cover the sentinel.
- cli.ts: add `-r`/`--random` plus an exported `selectPattern` holding the
  conflict rule. `-r` is sugar for `--pattern random`, so the two agreeing
  is a no-op while `-r -p arc` is rejected as contradictory. The flag folds
  into `pattern`, so ConfigOverrides, resolveConfig, and move.ts are
  untouched. `parseCliArgs` now takes its argv as an optional parameter so
  the flag surface is testable without process.argv.
- keeper.ts: resolve `random` via the picker once per trigger, before the
  loop-mode branch, so a pick holds for a whole loop run rather than
  changing mid-run. runKeeper builds one picker for the process, so the
  no-repeat memory spans sweeps minutes apart. Because the pick is a real
  strategy, --verbose logs the concrete pattern name and a pick with an
  infinite loopPath still bounces edge-to-edge under --loop.
- config.ts / configFile.ts: accept the sentinel where a pattern is valid,
  and quote the selectable list in errors. No `random` boolean config key —
  the file spells it "pattern": "random".

executor.ts and move.ts needed no changes.

Tests: new tests/cli.test.ts (the file had no coverage before) covering the
flag surface and the conflict rule; picker tests pinning the no-repeat and
full-registry-coverage properties; keeper tests pinning once-per-trigger and
once-per-loop-run.

docs/execution-happy-path.md

@@ -37,14 +37,14 @@ sequenceDiagram
         Note over Keeper: pos == lastPos (no user movement)<br/>now - lastActivity ≥ moveInterval → fire
     end
 
-    Keeper->>Sim: simulateActivity(config, log, dev)
+    Keeper->>Sim: simulateActivity(config, log, dev, pickRandom)
     Sim->>Dev: width()
     Dev-->>Sim: width
     Sim->>Dev: height()
     Dev-->>Sim: height
     Sim->>Dev: getPosition()
     Dev-->>Sim: start
-    Note over Sim: strategy = STRATEGIES[config.pattern]<br/>ctx = { start, width, height, rng }
+    Note over Sim: strategy = STRATEGIES[config.pattern]<br/>(or pickRandom() when pattern is "random")<br/>ctx = { start, width, height, rng }
     Sim->>Exec: executePath(strategy, ctx, dev, log, config)
 
     Exec->>Strat: path(ctx)