chore(tracking): remove repo-local beads state

[christso]

Summary

• remove tracked repo-local .beads state from the AgentV repository
• ignore local task-tracker state so it is not accidentally recommitted
• update contributor guidance to keep tracker state out of commits unless explicitly requested

Verification

• confirmed current origin/main repo-local tracker export has no tracked-only or newer records relative to the central tracker before deletion
• git diff --check
• targeted rg checks for stale repo-local .beads / Beads sync guidance
• git ls-files | rg '(^|/)\\.beads(/|$)' || true\n- bunx biome check biome.json CONTRIBUTING.md\n\nCentral task: av-3vw

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