Add bug Gate 3a for exhausted hypothesis tree

[developzoneio]

What

/sd:bug Phase 3 told the investigation loop to "Continue until one is CONFIRMED" — with no exit. A bug whose every hypothesis is rejected or inconclusive had no defined stopping point, and Gate 3 only fires on a confirmed root cause.

This adds Gate 3a (hypothesis tree exhausted):

• The loop now terminates on either a CONFIRMED hypothesis (→ Gate 3) or an exhausted tree — all rejected, or only inconclusive ones left with no new evidence (→ Gate 3a).
• Gate 3a STOPs and refuses to guess a fix from an unconfirmed tree. It records the exhausted tree to 03-decisions.md and asks the user to re-enumerate (with new evidence), observe (add observability, reproduce, re-enumerate), or abort as "root cause not found".

Mirrors the existing Phase 2 repro-refusal voice (commands/bug.md "REFUSE to proceed … Options:").

Scope

• commands/bug.md — Phase 3 loop termination + new Gate 3a.
• CHANGELOG.md — one ### Fixed line under [Unreleased].

Item 4a of the command/agent discipline refinements backlog.

Verification

bash scripts/validate.sh — all 6 checks pass.

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