Back-fill linear_issue_id on older blocked PR-shepherd fleet-tasks

Per LEARNINGS 2026-05-29 04:10 UTC Action Item #6. Recent direct-ask tasks have Linear keys recorded at task start (BOLT-597 / BOLT-1053 / BOLT-1054 / BOLT-1055). Older blocked PR-shepherd tasks may not. Targets: a8ef2296 (#9886), d2d63920 (#9932), f24f2035 (#9353), d18e5573 (#10508), 0e5f88e9 (#10401), 923743f4 (#10498), 3116b189 (#10499). DoD: for each task, (a) GET the task; (b) if linear_issue_id is null/empty, gh pr view <pr> to extract Linear key from title/branch/body (typical patterns: ALL-XXX, BOLT-XXX in the PR title or branch name); (c) PATCH task with linear_issue_id; (d) if no Linear key can be found, leave linear_issue_id empty but PATCH task description to note Investigated 2026-05-29: no Linear key in PR title/branch/body. (e) Report summary in result.