Remove deprecated channel_lan network + CHANNEL_NAME marker (portals rename cleanup)

[itlackey]

Follow-up to #436 (channels → portals rename, completed in 0.12.0). Two back-compat shims were retained for one release and should be removed in 0.13.0:

• channel_lan Docker network — removed from .openpalm/config/stack/core.compose.yml (it is currently a deprecated empty bridge, no first-party service joins it). Removing it will break any user custom.compose.yml overlay still referencing channel_lan; the 0.12.0 CHANGELOG already warns users to migrate to portal_net.
• CHANNEL_NAME compose marker — drop from PORTAL_MARKER_KEYS in packages/lib/src/control-plane/portals.ts (only PORTAL_NAME recognized). Safe once the 0.12.0 migration has renamed all on-disk compose assets.

Optional: a read-only startup scan that emits a deprecation note if channel_lan is found in a loaded compose overlay, to warn users before the 0.13.0 hard removal.

[itlackey]

Verification pass on synced main after PR #559 merge (c1eadbd30e107f76b71aa933b71edcaedaadba46).

Current state:

• packages/skeleton/system/stack/core.compose.yml still defines the deprecated channel_lan compatibility bridge and labels it as removed in 0.13.0.
• packages/lib/src/control-plane/portals.ts still recognizes both PORTAL_NAME and legacy CHANNEL_NAME in PORTAL_MARKER_KEYS.
• CHANGELOG and review docs still discuss the transition state.

Remaining work to complete this issue:

• Remove channel_lan from the managed skeleton compose files once the upgrade safety net is in place.
• Remove CHANNEL_NAME from portal marker detection so only PORTAL_NAME is canonical.
• Add a preflight or migration safety path for user custom.compose.yml overlays that still reference channel_lan, otherwise compose validation can hard-fail on upgrade.
• Update docs/CHANGELOG to remove the one-release compatibility wording or replace it with final migration instructions.
• Add grep/compose tests that prove no first-party compose or lib code still depends on channel_lan or CHANNEL_NAME.

Suggested verification gate:

• docker compose config passes for a normal current skeleton.
• An old-home fixture with custom.compose.yml referencing channel_lan gets a clear migration error or automatic rewrite before managed compose drops the network.
• PORTAL_NAME custom portals still discover correctly; CHANNEL_NAME no longer counts as a portal marker.