docs: add benchmark harness spec, ADRs, and task breakdown

Add the benchmark-harness spec, three supporting ADRs (cost-equivalent
token metric, single-user seed, guard-based tool isolation), and the
0022-0030 task breakdown that slices the harness into foundational
seams, an integrating single-cell runner, and the reporting layer.

.claude/tasks/0029-bench-run-loop-cli.md

@@ -0,0 +1,17 @@
+---
+spec: benchmark-harness
+blocked-by: [0027-bench-single-cell-runner, 0028-bench-task-suite]
+---
+
+## What to build
+
+The maintainer-facing command that runs a chosen benchmark cell on demand, so only the token budget available at that moment is spent. The maintainer selects an arm and a task; the command runs that cell and accumulates results.
+
+Each cell defaults to five trials with a reporting floor of three. Because results are immutable timestamped samples, running a cell that already has samples deepens it — the new trials append rather than overwrite, so a cell's sample size can be grown opportunistically across separate sittings.
+
+## Acceptance criteria
+
+- [ ] The command runs a single selected `(arm, task)` cell on demand.
+- [ ] A cell defaults to five trials, and the reporting floor of three is respected.
+- [ ] Re-running an already-sampled cell appends new trials rather than overwriting prior samples.
+- [ ] The command drives the runner and store built in earlier slices rather than reimplementing orchestration.