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.

[ivklgn]

Re-checked against v0.7.0: still open, nothing implemented. cmd/mcp.go still points one server instance at one root through --project / ARCHCORE_PROJECT_ROOT.

.archcore/cli/multi-project-mcp-access.idea.md still lists the same three undecided directions — per-project servers, a project_root parameter, attach_project tools — so this RFC is unstarted rather than stale.

One inconsistency worth fixing alongside it: the idea carries status: accepted while its implementation section says "several directions, to be evaluated later". The status reads as a settled decision the document does not contain.