feat: implement issue assignment agentic workflow (go:yes trigger) (#186)

* feat: implement issue assignment agentic workflow

Rewrite squad-issue-assign.yml to implement the go:yes label-triggered
assignment workflow:

- Trigger: issues labeled with go:yes (names filtering)
- Assigns issue to the maintainer who applied the label (sender)
- Reads prior triage analysis comment and .squad/routing.md
- Applies squad label + matched squad:{member} labels
- Safe outputs: assign-to-user max:1, add-labels allowed:[squad, squad:*]
  blocked:[go:*, priority:*, override:*, type:*] max:5, add-comment max:1
- Post-check job verifies assignee == sender and all squad:{member}
  labels exist in routing.md

Move the Routing Table and Issue Routing markdown tables from
.squad/routing.md into dedicated JSON files:
- .squad/routing-table.json (workType, routeTo, examples array)
- .squad/issue-routing.json (label, action, who)
- routing.md now references these JSON files. The squad-issue-assign
workflow reads JSON directly instead of parsing markdown tables,
resulting in much simpler and more reliable code.

* remove: delete squad-triage workflow

Remove the squad-triage workflow that auto-triaged on the 'squad'
label. The issue assignment workflow now requires maintainers to
explicitly apply 'go:yes' to approve and assign issues, enforcing
human-in-the-loop approval before any routing happens.

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: 4 people