Ports #7083 into the v2 session path, which replaced the v1
ProviderService where main's gate lives. Instead of withholding the whole
t3-code MCP credential — on this branch it also carries the thread
orchestration and worktree toolkits — the credential is minted without
the "preview" capability when enableAgentBrowserAccess is off, so every
preview tool call rejects while orchestration stays available.

ProviderSessionManager reads the setting at prepare time (deny on an
unreadable settings file, matching main), rotates a reused credential
whose capability set no longer reflects the setting, and the session
config now carries browserToolsAvailable so the Codex adapter keeps its
developer instructions truthful via main's parameterized instruction
builders instead of the removed constants.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>