feat(pi-sentinel): make sentinel creation idempotent

Re-registering a same-name sentinel with an equivalent spec now returns
the existing sentinel as a success instead of failing with
'Sentinel <name> already exists'. A different spec still fails, showing
both specs and suggesting replace: true, which swaps the sentinel.
The concurrent stream-watch limit error is unchanged.

11 of 20 live tool errors in 14 days were 'already exists' noise from
agents re-establishing watches they already held.