fix(subos): decouple storage from sandbox + transparent bwrap probe errors

[Sunrisepeak]

Summary

V6 storage isolation auto-forced --sandbox entry when storage=image|tmpfs, fusing two axes that V4 design kept orthogonal. Result: xlings subos use <image-subos> silently required root + bwrap + working mount namespace just to switch shells, and any failure cascaded into a run: xlings install bwrap loop with no real diagnostic.

This PR is a UX hotfix covering four items from .agents/docs/sandbox-v6-followup-fixes-2026-05-15.md:

• P0-1 — Decouple storage from sandbox: subos use <name> without --sandbox always enters shell-level regardless of storage. Image / tmpfs only take effect on the explicit --sandbox path. New warn_storage_dormant_on_shell_ helper emits a one-line stderr hint (eval-safe for --shell <kind> output mode).

• P0-3 — probe_bwrap_ transparency: replaced system() + 2>/dev/null + bool with platform::run_command_capture + platform::shell_quote, returning (ok, output). New classify_bwrap_probe_error_ matches three known failure modes (setuid build-mode mismatch, AppArmor uid_map block, kernel-disabled userns) and returns actionable hints; unmatched output falls through to raw stderr so users always see the truth.

• P0-3 (continued) — Re-probe on image-rejects-proot: when image/tmpfs hits the proot fallback rejection, re-probe bwrap and surface the classified error instead of the stale xlings install bwrap suggestion.

• P0-4 — subos remove umounts image first: detect a live <subos>/.mountpoint and unmount_image_ before rm -rf. Without this, rm -rf either hit EBUSY at the mountpoint or recursed into the live filesystem and erased image contents (home.img marked (deleted)). Linux-guarded.

Cross-platform impact

• 🐧 Linux: image/tmpfs storage no longer touches sandbox code path on shell-level entry; bwrap probe failures now show real causes; remove safely umounts before rm.
• 🍎 🪟 macOS/Windows: shell-level path unchanged (env/PATH only); image/tmpfs storage was already unsupported on these platforms, P1-6 (cross-platform storage validation on subos new) is tracked as a separate followup.

User-facing behavior change

Before:

$ xlings subos use mode-image-test
[xlings] storage=image requires sandbox, entering sandbox mode...
[sudo] password for speak:
[error] image storage requires bwrap (proot does not support mount namespace)
[error]   run: xlings install bwrap     ← misleading, bwrap was already installed

After (with bwrap broken/missing on host, but user wanted shell-level only):

$ xlings subos use mode-image-test
[xlings] storage=image is sandbox-only; entering shell-level (use --sandbox to activate)
<xsubos:mode-image-test>$           ← directly enters, no sudo, no bwrap touch

After (when user explicitly does --sandbox but bwrap build is broken):

$ xlings subos use mode-image-test --sandbox
[error] image storage requires bwrap (proot does not support mount namespace)
        xim:bwrap binary was built without setuid support; rebuild xlings-res/bwrap mirror with `-Dsupport_setuid=true` and bump the version
          binary: /home/speak/.xlings/data/xpkgs/xim-x-bwrap/0.11.2/bin/bwrap

Test plan

• xmake build xlings — clean compile (subos.cppm rebuilt, no errors/warnings on changed code)
• bash tests/e2e/subos_sandbox_test.sh — passes (S1-S2 verified; S3+ skipped pending live backend, which is the existing behavior)
• bash tests/e2e/subos_install_remove_isolation_test.sh — passes (remove path unchanged for shared-storage subos)
• bash tests/e2e/subos_workspace_c2_schema_test.sh — passes (workspace schema unaffected)
• Manual: image-storage subos + subos use --shell sh — stdout is eval-safe (exports only), stderr has the storage hint, eval "$out" then echo $XLINGS_ACTIVE_SUBOS returns the subos name
• Manual: image-storage subos + subos remove (no live mount) — succeeds cleanly, directory removed
• Manual: shared-storage subos + subos use --shell sh — no storage hint emitted (correct)
• CI: full Linux/macOS/Windows/ArchLinux matrix

Followups (separate PRs)

Tracked in .agents/docs/sandbox-v6-followup-fixes-2026-05-15.md:

• P1-5: scan stale .mountpoint on sandbox entry (kill -9 / force-close recovery)
• P1-6: subos new --storage image|tmpfs cross-platform validation (reject at create time on macOS/Windows)
• P2-7: xim:bwrap install hook sudo prompt UX