[pkg firewall] add the VS Code product.json patch lifecycle

[dekkagaijin]

Merge after #25

Second code chunk. Builds on the JSON primitives to give discovery, a three-state machine, and patch/restore. Still nothing calls it — the generator wiring is a later change — so this can be reviewed as the state machine it is.

vscode_install_paths stable and Insiders, macOS/Linux
vscode_edition_label from nameLong, so output names the edition
vscode_is_readonly_install snap/flatpak by path prefix
vscode_can_write probes without modifying
vscode_node_bin VS Code's own Electron, via CFBundleExecutable
vscode_managed_state unmanaged | current | stale
vscode_marker_field reads the marker, single-line or pretty-printed
vscode_patch / vscode_unpatch
vscode_patch_via_node / vscode_unpatch_via_node

The patch is one key set and one key removed:

serviceUrl -> the firewall's gallery URL. VS Code derives both
${serviceUrl}/extensionquery and
${serviceUrl}/vscode/{publisher}/{name}/latest from it.
extensionUrlTemplate -> removed. It is the fallback when the resource API
returns 5xx. Left in place, a firewall outage
silently resolves versions from www.vscode-unpkg.net,
bypassing the firewall exactly when it is unhealthy.
Removed, that failure retries our own extensionquery
and fails the install if that also fails.

controlUrl (Microsoft's malicious-extension revocation list), resourceUrlTemplate, itemUrl, publisherUrl, nlsBaseUrl, mcpUrl and accessSKUs are left alone, and the merge is key-level, so keys future VS Code versions add survive untouched.

Three states, because credentials rotate:

unmanaged marker absent -> capture the original, then patch
current marker + expected line -> no write at all
stale marker but not current -> restore first, then patch

Never patching on top of a patch is the invariant that lets credentials rotate indefinitely without the captured original drifting.

Failure modes are loud rather than silent, because a silent no-op is indistinguishable from success in an MDM log:

• An unwritable product.json on macOS almost always means the missing App Management (TCC) grant, and says so. Root is not exempt from that gate.
• snap/flatpak installs are refused by path prefix, not by [[ -w ]] — under root that test returns true on a read-only mount.
• Not line-oriented (repackaged, minified) falls back to VS Code's own bundled Electron as node. Never hardcodes "Electron": stable ships "Code".
• --dry-run redacts the _ak token. A deliberate deviation from the other ecosystems, which echo full credentialed URLs: this one is a bearer credential in a URL path, and MDM consoles retain output for more people than can read product.json.

Tests: 60 assertions covering the full state machine, both writer paths, the EPERM/TCC message, the snap refusal, dry-run redaction, and byte-exact restore after two patch cycles. Also patches and restores whichever real product.json is installed on the machine, asserting nothing version-specific.

https://endorlabs.atlassian.net/browse/LM-452