feat(training-agent): add /si tenant for Sponsored Intelligence lifecycle (#3940)

[bokelley]

Summary

Fixes #3940. Adds the /si training-agent tenant so learners can practice the full Sponsored Intelligence session lifecycle against a deterministic training brand (Nova Brands).

• New tenant (tenants/si.ts + v6-si-platform.ts): registers all four SI tools via the customTools merge seam (same pattern as update_rights/creative_approval in the brand tenant — SDK DecisioningPlatform has no SI interface field yet)
• Tool catalog (tenants/tool-catalog.ts): adds si_get_offering, si_initiate_session, si_send_message, si_terminate_session under ['si']
• Registry (tenants/registry.ts): registers the SI tenant alongside the existing six; updates module comment from "Six" to "Seven"
• Migration 465 (db/migrations/465_si_tenant_module_pins.sql): pins tenant_ids for A3, C3, and S5 (left NULL in migration 464 pending this tenant); fixes phantom tool connect_to_si_agent → si_initiate_session in C3's c3_ex2; appends missing si_get_offering/si_terminate_session sandbox actions to S5's s5_ex1; stamps stable criterion IDs s5_ex1_sc_session_lifecycle and s5_ex1_sc_offering_integration via _append_criterion
• Drift test (tests/integration/training-agent-tool-catalog-drift.test.ts): adds 'si' to TENANT_IDS
• Smoke test (tenants/tenant-smoke.test.ts): adds 'si' to expected agent IDs

Implementation notes

• Session state is in-memory per process — acceptable for the sandbox; sessions are training-scoped and not persisted across restarts
• si_get_offering response matches the JSON schema shape: { available, offering_token, offering: { offering_id, title, summary }, matching_products: [...] }
• si_terminate_session marks sessions as terminated: true in-map (does not delete) so a subsequent si_send_message returns SESSION_TERMINATED rather than the misleading SESSION_NOT_FOUND
• si_terminate_session response includes the required terminated: true field per si-terminate-session-response.json schema
• Session IDs use crypto.randomUUID() for sufficient entropy

Human task required

.github/workflows/training-agent-storyboards.yml storyboard floor assertions are not updated by this PR — agent policy prohibits editing .github/**. A human should add SI storyboard floors once the tenant is exercised in CI.

Test plan

• pnpm test --filter server -- training-agent-tool-catalog-drift — drift detection passes for all 7 tenants
• pnpm test --filter server -- tenant-smoke — smoke test passes with si in the agent list
• Manually call tools/list on /si/mcp and verify four SI tools appear
• Manually exercise the full lifecycle: si_get_offering → si_initiate_session (passing offering_token) → si_send_message → si_terminate_session; verify terminated: true in response
• Verify si_send_message after terminate returns SESSION_TERMINATED error code
• Run migration 465 against a dev DB and confirm A3/C3/S5 tenant_ids are populated
• Confirm C3 c3_ex2 no longer references connect_to_si_agent
• Confirm S5 s5_ex1 sandbox_actions includes all four SI tools

https://claude.ai/code/session_01SAU73ibxYgk3pdvmMJNjcF

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Generated by Claude Code

[bokelley]

Holding for rebase. The /si tenant is still wanted and not duplicated by any merged PR, but main has had ~30+ commits to server/src/training-agent/ since this branch was cut (#4060–#4073, #4098, #4099, #4326, #4346 among others), so the rebase needs to thread through several recent changes:

Hard conflict:

• Migration number collision — 465_si_tenant_module_pins.sql clashes with 465_founding_member_audit_columns.sql on main. Renumber to the next available slot (probably 476+ given current main) and re-verify migration 464_certification_module_tenants.sql still leaves the relevant tenant_ids NULL as this PR's setup assumes.

Soft conflicts (likely):

• Tenant registry — tenants/registry.ts + tenants/tool-catalog.ts have been touched repeatedly. Slot the /si registration alongside the six current tenants.
• v6 platform brand-threading — per the v6 SalesPlatform shim brand threading pattern established in feat(training-agent)!: migrate to @adcp/sdk@6.0.0 + split into per-specialism tenants #3713 / fix(training-agent): restore per-tenant /<tenant>/mcp-strict, close #3965 Class C #4061, the new v6-si-platform.ts shim methods need to thread ctx.account.ctx_metadata.brand_domain into args, otherwise session-keying drops to open:default and test fixtures silently miss.
• Post-fix(training-agent): tenant router lock + storyboard-smoke --brand #4098 transport-binding — fix(training-agent): tenant router lock + storyboard-smoke --brand #4098 added a per-tenant async lock around the shared Server's connect → handle → close lifecycle. The /si tenant mount needs to use the same lock pattern (see the existing /mcp-strict routes added in feat(training-agent): mount /<tenant>/mcp-strict-required + mcp-strict-forbidden, route signed_requests at all three #4099 for the current shape).

Failing CI: Server integration tests log was truncated to setup phase — could be flake against the stale base or a real registry-init regression. Re-run post-rebase to see what actually fails.

Recommended path: re-trigger via /triage execute on #3940 (the source issue), or check out the branch locally, do the migration renumber + registry merge + v6 shim brand-domain threading by hand. Estimated 1–2 hours of focused work. Not closing — this is a real feature gap, just blocked on a substantial rebase.