Rule star's session-root anchor #571

Design question, split out of #568 step 3, where deleting the unconfined fsroot variant forced star's
session root to name an anchor for the first time.

The question

star builds its runtime environment with the session root at the working directory, unconfined
(cmd/star/star/application.go). With one confinement behaviour, that root must anchor somewhere — and the
choice encodes what star is. Star's charter pulls in two directions: "extensible build tool that operates
anywhere" wants the widest address space; the hermeticity direction (#563, module subsets) wants the narrowest
one the script declares.

A hard constraint either way: star must read and write the XDG trees, and XDG locations are only
conventionally under home — XDG_CONFIG_HOME/XDG_STATE_HOME overrides can point anywhere, so every fixed
anchor except the filesystem root can be escaped by a legitimate XDG configuration.

Candidates

1. Current directory — what make/go build/bazel mean by context, and the narrowest. Fails: XDG
   unreachable; invoked from a subdirectory it cannot see the repo root; any "read a file from over there"
   script breaks.
2. Repository root — the better build-tool mental model. Fails: XDG still unreachable; undefined outside a
   repo, so a fallback rule reintroduces the whole question one level down.
3. Home — covers default XDG and most working trees. Fails: confinement theater — nearly everything the
   user owns is under home — while still breaking repos in /srv or /opt, XDG overrides outside home, and a
   Windows working tree on a different volume than the profile.
4. Multiple roots — a work root (cwd or repo) plus XDG trees opened separately; roots as declared
   capabilities, the same shape as Every runtime loads every module, so planning can call file and platform immediately #563's module subset. The design already endorses it ("cross-tree work wants
   two confined roots rather than one granting unbounded reads"), and devlore's own config/state/cache access
   already flows through dedicated roots (cli.OpenTree at the XDG anchors), not the session root. Fails only
   in cost: the runtime environment mints exactly one root today, so this is a design effort, not an edit.

Interim (shipped with #568 step 3) — REVISED

Star anchors at the working directory, confined. The volume-root interim originally recorded here was
tried and failed: star scripts resolve relative paths against the session root's anchor, so a volume
anchor silently redefined every relative path in every script — the generator's go.mod walk
(compute_provider_import in generate.star) broke immediately. "Preserves today's behaviour" was true only of
absolute paths.

wd-confined preserves relative-path semantics — the dominant use; the entire generator toolchain — and drops
only unconfined absolute access, which no current star extension exercises (verified: no absolute filesystem
paths, no XDG references in star/extensions). Residuals accepted until this issue is ruled: XDG trees are
unreachable through the session root, and star invoked from a subdirectory cannot see the repository root —
both listed under candidate 1 above.

Where this must be answered

No later than #563's hermeticity design. If roots become declared capabilities there, this question dissolves
into it — option 4 is that design. Until then the volume-root interim holds, unless a Windows cross-volume
star use case forces it earlier.