docs(agents): externalize task tracker guidance

[christso]

Summary

AgentV's public worker guidance now treats task tracking as operator-supplied context instead of telling every worker to use repo-local Beads. If a prompt supplies an external tracker database, path, or environment variable, workers use that source; otherwise they avoid creating, syncing, staging, or committing local tracker state.

This also removes the conflicting auto-inserted bv workflow block and keeps the reusable public guidance around Agent Mail reservations, worktrees, GitHub PRs, and handoff state.

Verification

• Red: initial rg found root AGENTS.md instructions for Beads as the normal task graph, br sync --flush-only, staging .beads/, and the auto bv-agent-instructions-v2 block.
• Green: rg -n "Beads|beads|\\.beads|BD_DB|\\bbr\\b|\\bbv\\b|br sync|br update" AGENTS.md returns no matches.
• git diff --check
• Pre-push hook passed: bun --filter @agentv/core typecheck, bun --filter @agentv/phoenix-adapter typecheck, bun --filter agentv typecheck, and biome check .

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