fix(reborn): mount per-task workspace at /workspace + use scoped paths

[pranavraja99]

Problem

Reborn pinchbench/clawbench tasks that use coding tools (write_file, read_file, list_dir, shell) failed with dispatch_failure_kind=InputEncode → HostUnavailable{Capability} → terminated turn (empty response, 0 score). 15/26 pinchbench tasks were affected; reproduced on both DeepSeek-V4-Flash and Qwen3.5-122B.

This was initially misattributed to an ironclaw main regression (approval gates). It is not — ironclaw is innocent; the bug is entirely bench-side. Proven: on the same new-main ironclaw, an absolute workspace path → 3 InputEncode; a scoped /workspace path → 0, dispatch completes.

Root cause

resolve_workspace_placeholder resolved {{WORKSPACE}} to the absolute host dir (ws_base/task_id) and the prompt told the model to use absolute paths. But reborn's coding tools scope paths to the /workspace mount and reject absolute host paths (coding/paths.rs::resolve_path → input_error() = InputEncode) — even under reborn-yolo, tools do not take raw host paths. (The old-fork passing run had {{WORKSPACE}} unresolved, so the model used /workspace/... and it worked.)

Second, the reborn /workspace mount defaulted to {local_dev_root}/workspace, which the bench never seeds or scores — so even valid scoped writes landed where the grader couldn't see them, and seeded inputs were invisible to the agent.

Fix

• run_reborn_conversation gains workspace_root: Option<&Path> and threads it into RebornBuildInput::with_local_dev_workspace_root, mounting the per-task workspace dir (ws_base/task_id — the exact dir the suite seeds inputs into and the scorer reads from) at /workspace.
• run_reborn_task resolves {{WORKSPACE}} → /workspace (scoped) instead of the absolute host dir. The legacy host-fs path keeps the absolute form (its tools take raw host paths).
• Standalone reborn CLI command passes None (unchanged behavior).

Verification

task_03_blog (reborn, against current ironclaw main):

• 0 InputEncode (was 3) — coding tools dispatch cleanly.
• blog_post.md now lands in workspaces/task_03_blog/ (the scorer's dir) — previously orphaned in reborn-task_03_blog/workspace/.

cargo check passes against the pinned ironclaw branch. No ironclaw change required.

🤖 Generated with Claude Code

[pranavraja99]

Superseded by #108 — the workspace-mount fix (a19f428) is folded into #108 on top of the ironclaw-main repin + API sync, since they overlap in reborn_runner.rs/runner.rs and #97's old API (with_provider) no longer builds against ironclaw main. Verified there: pinchbench reborn now scores real values (task_01_calendar 0.833, task_03_blog 0.92) instead of 0/'pending'. Closing in favor of #108.