RFC: cross-project MCP access from a single session

[ivklgn]

Problem

Today the --project flag / ARCHCORE_PROJECT_ROOT points one MCP server instance at one project directory. Reaching multiple projects from a single session — the actual goal of this idea — is unimplemented, and the idea lists three undecided directions.

Expected result

An RFC that evaluates the options and specifies a concrete approach (e.g. a project_root tool parameter or an attach_project tool), including the path-isolation/security model, recorded as a decision.

Impact

Unblocks a well-scoped implementation issue for cross-repo context access instead of building against an undecided design.

Scope

• Evaluate the three directions; recommend one.
• Specify the tool/parameter surface and the security/path-isolation model.
• Record the decision and update the idea's status.

Files: a new RFC/decision doc under .archcore/; references cmd/mcp.go

Acceptance: a decision is recorded and an implementation issue can be split off.

Status: design/RFC only — no implementation in this issue.