fix(mcp): fall back to cwd basename on ambiguous project (#248)

[CarlosPlasencia]

Summary

Closes #248. Restores v1.12.0 behavior for MCP write tools launched from a directory that is the parent of multiple git repositories.

When the MCP server is launched from a workspace folder containing several cloned repos, all write tools (mem_save, mem_session_summary, mem_session_start, etc.) currently fail with:

{ error_code: "ambiguous_project",
  message: "ambiguous project: multiple git repos found in cwd",
  available_projects: [...] }

This is a regression from v1.12.0 → v1.13.0 introduced by the mcp-project-autodetection change, which switched resolveWriteProject from the basename-fallback CLI helper to the strict DetectProjectFull and propagated ErrAmbiguousProject unconditionally.

What changed

internal/mcp/mcp.go — resolveWriteProject:

• On ErrAmbiguousProject, fall back to filepath.Base(cwd) lowercased — same behavior as the CLI wrapper DetectProject (internal/project/detect.go:224).
• Set Source = SourceDirBasename.
• Emit a Warning listing the cwd repo children so the caller knows the project name was derived from the workspace folder, not from a specific repo.
• Non-ambiguous cases are untouched (single repo, git remote, git child promotion, etc.).

Test changes

Five tests were rewritten to assert the new fallback contract instead of the old fail-fast contract:

Old test (asserted error)	New test (asserts fallback success)
TestMemSave_AmbiguousEnvelope	TestMemSave_AmbiguousFallback
TestResolveWriteProject_AmbiguousError	TestResolveWriteProject_AmbiguousFallback
TestMemSessionSummary_AmbiguousReturnsError	TestMemSessionSummary_AmbiguousFallback
TestWriteTool_AmbiguousErrorUsesCwdRepos_NotAllProjects	TestWriteTool_AmbiguousFallbackWarningListsCwdRepos
TestHandleGetObservation_DegradedPathNoEnvelope	TestHandleGetObservation_AmbiguousFallbackEnvelope

Each new test verifies: (1) write succeeds, (2) project_source = "dir_basename", (3) warning is present, (4) for the JW1 test, the warning lists cwd repos and not unrelated store projects.

Full test suite passes locally on macOS arm64 (Go 1.26.2):

ok  github.com/Gentleman-Programming/engram/cmd/engram
ok  github.com/Gentleman-Programming/engram/internal/cloud (and 7 cloud subpackages)
ok  github.com/Gentleman-Programming/engram/internal/mcp
ok  github.com/Gentleman-Programming/engram/internal/obsidian
ok  github.com/Gentleman-Programming/engram/internal/project
ok  github.com/Gentleman-Programming/engram/internal/server
ok  github.com/Gentleman-Programming/engram/internal/setup
ok  github.com/Gentleman-Programming/engram/internal/store
ok  github.com/Gentleman-Programming/engram/internal/sync
ok  github.com/Gentleman-Programming/engram/internal/tui
ok  github.com/Gentleman-Programming/engram/internal/version

Spec note

The archived change 2026-04-24-mcp-project-autodetection (REQ-309) currently specifies fail-fast on ambiguous cwd. After this PR is merged, that requirement is no longer accurate for the runtime contract. A follow-up SDD change should update the spec to document the basename fallback (Source=dir_basename + non-empty Warning) so the spec and code stay aligned. Happy to open it as a separate PR if you prefer.

Test plan

• go test ./internal/mcp/... — passes
• go test ./... — passes
• Verified locally with mem_save from a workspace with multiple git children — write now succeeds with project_source: "dir_basename" and a warning listing the children.