test: build e2e yoi binary provider

.yoi/tickets/00001KV0TJVN5/thread.md

@@ -83,4 +83,33 @@ Escalate if:
 
 Ticket evidence、existing E2E harness code map、Orchestrator worktree clean state を確認した。ユーザーが明示的に修正を依頼しており、blocking relation はない。実装 side effect の前に inprogress acceptance を記録する。
 
+---
+
+<!-- event: implementation_report author: hare at: 2026-06-13T15:54:18Z -->
+
+## Implementation report
+
+Implementation report for Ticket 00001KV0TJVN5
+
+Files changed:
+- `tests/e2e/src/lib.rs`
+  - Added a cached e2e binary provider using `OnceLock`.
+  - Preserves `YOI_E2E_BIN=<path>` as the explicit override and skips the default cargo build provider in that path.
+  - Default path runs `${CARGO:-cargo} build -p yoi --features e2e-test --bin yoi` from the workspace root, then returns the direct `target/{profile}/yoi` binary path for PTY spawning.
+  - Writes `target/e2e-artifacts/binary-provider.json` and emits diagnostics with provider, build command, and binary path.
+  - Expanded command-failure diagnostics to include command args.
+- `tests/e2e/tests/panel.rs`
+  - Updated panel tests to use the fallible cached binary provider.
+
+Validation:
+- `cargo fmt --check` — passed.
+- `git diff --check` — passed.
+- `cargo check -p yoi-e2e --all-targets --features e2e` — passed.
+- `unset YOI_E2E_BIN && cargo test -p yoi-e2e --features e2e --test panel -- --nocapture` — passed; default provider built the current `yoi` binary and PTY-spawned `target/debug/yoi`.
+- `YOI_E2E_BIN=/home/hare/Projects/yoi/.worktree/e2e-binary-provider/target/debug/yoi cargo test -p yoi-e2e --features e2e --test panel -- --nocapture` — passed; override provider path used without invoking the default cargo-build provider.
+
+Remaining gaps:
+- None known.
+
+
 ---