fix(kanban): decomposed worktree children inherit the root's repo anchor

[jayleaton]

Problem

The auto-decomposer (decompose_triage_task) deliberately left workspace_path NULL on worktree-kind children so dispatch would materialize a fresh <repo>/.worktrees/<child-id> per sibling (sibling-isolation fix, #67567 lineage). But dispatch's fallback for an anchorless worktree task is the board's default_workdir — and when the board has none, _resolve_worktree_workspace raises:

task <id> has workspace_kind=worktree but no workspace_path, and board '<slug>'
has no default_workdir set.

Live manifestation (2026-09-02, engineering board): the auto-decomposition of t_47766623 produced reviewer/QA children with workspace_kind=worktree, workspace_path=NULL; both failed to spawn twice and hit the failure-limit circuit breaker (gave_up), blocking the whole downstream callback graph.

Fix

The decomposition root is usually itself a dispatcher-materialized worktree <repo>/.worktrees/<root-id>, so the repo is recoverable from the root row. decompose_triage_task now:

1. When the root's workspace kind is worktree with a path, resolves the root's repo anchor:
   • linked-worktree checkout → the parent of its git common dir (<repo>/.git → <repo>);
   • otherwise the containing repo toplevel.
2. Stamps that repo as each pathless worktree child's explicit workspace_path anchor. Dispatch still materializes a distinct <repo>/.worktrees/<child-id> + wt/<child-id> per child, so per-sibling isolation is unchanged (the anchor is the repo, never the root's literal checkout).
3. Falls back to the previous NULL row when no repo can be recovered (board-default dispatch path), and never touches scratch roots or per-child explicit overrides.

Tests

New regression coverage in tests/hermes_cli/test_kanban_worktree_isolation.py:

• worktree children inherit the root's repo anchor (not the root checkout) and each child resolves to its own worktree/branch end-to-end via _resolve_worktree_workspace with no board default_workdir — the exact previously-failing condition;
• unrecoverable root path → children keep the anchorless row;
• scratch-root decomposition stays scratch (no kind/path leakage).

scripts/run_tests.sh over the decompose/worktree/kanban-db/tools scope: 108 passed, 0 failed. ruff clean.

[jayleaton]

Review requested: this PR is ready for maintainer review. Scope: hermes_cli/kanban_db.py decompose_triage_task worktree-anchor inheritance + 3 regression tests in tests/hermes_cli/test_kanban_worktree_isolation.py. Verification: scripts/run_tests.sh over the kanban decompose/worktree/db/tools scope (108 passed, 0 failed), ruff clean. @NousResearch/maintainers please review — note the fork-review permission limitation prevented a formal reviewRequests entry (403 on RequestReviewsByLogin); this comment is the explicit review request. Do not merge on my behalf.