[plan-04] Installer Failure Transparency — cross-IDE detection & error taxonomy

Defect

The installer's host/IDE detection is heuristic and silent when wrong. An npm-global Claude Code install on Windows is mis-detected as the "desktop app", which routes the user down a path that breaks headless memory generation — with no diagnostic that the wrong host class was chosen. This is the installer-transparency contract gap: detection must be explicit, verifiable, and loud when it cannot prove the host class.

Children

• #2723 — Windows: npm-global Claude Code install mis-detected as "desktop app", breaking headless memory generation

Fix sequence

Design doc: plans/04-installer-transparency.md. Make host/IDE detection explicit and assertable; emit the detected class + evidence; fail loud (not silent) on ambiguous detection; cover the npm-global-vs-desktop discriminator in the install test matrix.

Test matrix

Host	Install method	Detected class
Windows	npm -g Claude Code	CLI/headless (not desktop)
Windows	Claude Desktop	desktop
all	ambiguous	loud diagnostic, no silent mis-route

Out of scope

Packaging/tarball contents (plan-10); spawn templating (plan-02).