fix(addie): stop Sage passing demo.example.com as brand_id in C2 demos

[bokelley]

Closes #4074

During certification module C2 (Brand Protocol), the system prompt unconditionally appended "Use brand domain "demo.example.com" for the account." for all active modules. When C2 was active, Sage interpreted demo.example.com as the brand_id for get_brand_identity, which expects roster IDs like daan_janssen — returning REFERENCE_NOT_FOUND and falling back to spec-only teaching.

Root cause: buildCertificationContext (line 557) appended the buyer-domain hint with no module guard. For brand-track modules, Sage conflated "brand domain" with get_brand_identity's brand_id parameter.

Changes:

• Remove the unconditional demo.example.com instruction from the top-level sandbox-agent block (line 557)
• In both start_certification_module and get_certification_module demo-scenario blocks (already gated on lp.demo_scenarios?.length): emit buyer-domain guidance only when acquire_rights/sync_accounts is in the scenario tools; emit brand_id-roster guidance only when get_brand_identity is in the tools
• De-hardcode the get_products reference in the generic teaching rule (line 510) so non-sales-track modules get correct tool guidance; add "if the module has demo scenarios" guard
• Replace no more get_products calls → no more sandbox tool calls in the mastery fast-track rule for consistency
• Bump CODE_VERSION → 2026.05.1 for the buildCertificationContext behavior change

Non-breaking justification: server-side Addie system-prompt change only; no schema, protocol, or wire format touched. Changeset is --empty.

Pre-PR review:

• code-reviewer: approved — flagged two blockers (missing guard in get_certification_module handler; CODE_VERSION not bumped), both fixed before this PR was opened. One nit (symmetry in .includes vs .some) surfaced in PR body.
• education-expert: approved — confirmed daan_janssen is the correct C2 canonical example; flagged one blocker (line 510 fires for modules with no demo_scenarios), fixed. IACET-relevant: broken demo constitutes mismatch between stated and delivered instruction.

Nits (not fixed, noted for follow-up):

• c2_ex3 success criteria don't include a step that calls get_brand_identity with authorized: true to demonstrate the available_fields pattern — minor criterion-coverage gap (separate issue)
• scenarioTools.includes('get_brand_identity') vs .some(t => t === 'get_brand_identity') — minor style inconsistency, not functional

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See #3121
for context.

Session: https://claude.ai/code/session_019qaFNRgQ5bht7PNJpgHmiR

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