Blackboard: drop wrong respawn line; restore bb_already_written for final respondent

Two defects found by bin-venus-cc smoke pass on the MSG-343 entry. (1) MSG-342 invite copy tells agents to respawn if they lack the blackboard tools; false for Claude Code, which loads tool schemas on demand via ToolSearch with no restart - the advice costs a full context resend for nothing. Drop the line. (2) MSG-343 regression: the final respondent's duplicate write returns bb_entry_closed instead of bb_already_written, because their first write completes the entry so the status gate fires before the PRIMARY KEY violation. Disambiguate inside the 0-changes branch. Diff by coder-llmmsgsrv-cc against hub.mjs md5 6b31ce19f2a392cb96f1579cfa9539f9.