feat(coding-agents): add opt-in project scoping within shared banks

[voarsh2]

Adds an opt-in tag-scoped mode for installations that deliberately keep multiple projects in one Hindsight bank.

This complements the new optInOnly / per-project routing support rather than replacing it:

• optInOnly controls which projects may use memory at all.
• Separate dynamic banks remain the default and provide strong isolation.
• This PR supports the different deployment shape where related opted-in projects intentionally share one established bank, while ordinary retrieval should stay project-local.

It builds on #3418, which supplies consistent ingestion attribution across coding-agent write paths.

Configuration

{
  "bankId": "team::shared",
  "projectScope": "tags",
  "projectTagTemplate": "project:{gitProject}",
  "globalTags": ["scope:global"]
}