feat(mcp): add all_projects flag to mem_search for cross-project search

[carlos-mora-sm]

📋 Pre-flight Checks

• I have searched existing issues and this is not a duplicate
• I understand this issue needs status:approved before a PR can be opened

🔍 Problem Description

When working on a project, it's common to need context from a related project — a decision made last week in a different repo, a pattern established in another service, or a known gotcha from a dependency project.

Currently, mem_search always restricts results to the current/default project. There's no way to reach across projects from within the MCP tool.

The CLI already supports this via engram search "query" (without --project), which searches globally. The MCP tool has no equivalent.

Real scenario: while working on project-A, I need to recall an architectural decision logged under project-B. Today this requires either multiple mem_search calls with explicit project values, or falling back to the CLI outside the agent context.

💡 Proposed Solution

Add an optional boolean parameter all_projects to mem_search.

Example usage:

mem_search(query: "auth middleware decision", all_projects: true)

When all_projects: true, the tool skips the default project assignment and passes an empty project to Store.Search(), which already handles global search correctly (no WHERE project = ? clause is added).

When all_projects is false or omitted, behavior is unchanged.

📦 Affected Area

MCP Server (tools, transport)

🔄 Alternatives Considered

Passing project: "" explicitly — doesn't work because the handler replaces empty project with cfg.DefaultProject before calling Store.Search().

📎 Additional Context

Root cause is in internal/mcp/mcp.go, handleSearch() (~line 634):

if project == "" {
    project = cfg.DefaultProject  // always overwrites empty project
}

The CLI (cmd/engram/main.go, cmdSearch) doesn't apply this default, so opts.Project stays "" and Store.Search() performs a global query.

The store layer already supports global search — Store.Search() only adds WHERE o.project = ? when opts.Project != "" (internal/store/store.go, ~line 1536).

The fix is minimal:

if project == "" && !allProjects {
    project = cfg.DefaultProject
}