FDR-0019: support //name system-scope repos (drop the XDGSystem CheckSupported reject)

[friedenberg]

Context

repo_id.CheckSupported (go/internal/bravo/repo_id/main.go) currently rejects
XDGSystem-location ids (//name). Dropping that reject to actually support
system-scope repos needs work in both madder and dodder — it is not a pin
bump. Split out of #274 (which is now just the safe EffectiveName delegation);
deferred per maintainer decision (system is the least-used scope).

Why it's blocked (case b)

Dodder deliberately routes cwd/system-location ids into madder's
MakeDefaultAndInitialize via repo_id.EffectiveId (which preserves the
location type) — see command_components_dodder/env_repo.go:42-55. The comment
there is explicit: system is gated by CheckSupported "to keep the explicit 501
rather than silently resolving to the user tree if the gate is ever relaxed." So
a //name id reaching madder's MakeDefaultAndInitialize panics
Err501NotImplemented for LocationTypeXDGSystem today.

Required work

1. madder: wire XDGSystem resolution into MakeDefaultAndInitialize
   (go/internal/echo/env_dir/construction.go) so a system-location id resolves
   to the system root instead of 501-ing. Add JSON content negotiation for read-only HTTP API clients #230 shipped //name for the
   serve/discovery/system-temp paths but not this constructor. (madder owner has
   offered to pick this up — the symmetric "resolve LocationTypeXDGSystem"
   change.)
2. dodder: make the local working copy path scope-aware. MakeLocalWorkingCopy
   (command_components_dodder/local_working_copy.go) uses
   env_dir.MakeDefault(EffectiveName), which drops the location — so even after
   madder's fix, //name would mis-resolve to a user repo on the
   show/query/edit path. It needs the same location-aware branch
   env_repo.go already has (cwd/system -> MakeDefaultAndInitialize(EffectiveId)).
3. Then drop the XDGSystem reject in repo_id.CheckSupported and offer //name
   in -repo_id completion.

Dependencies

• madder pin must include the XDGSystem MakeDefaultAndInitialize wiring (a tag
  past go/v0.3.42).
• Coordinate with the madder owner before starting.

:clown: via clown 0.3.12+5220e30 — amarbel-llc/clown@5220e30