docs(lanes): document ccam lanes sync-base (E2)

docs/LANES.md

@@ -1013,6 +1013,28 @@ QC_BOOT_ENV="MOCK_PAYMENTS=1 STUB_EMAIL=1"
 
 These environment variables are passed to the profile's `boot` hook during `ccam lanes up --qc`, ensuring QC test runs are deterministic.
 
+### Dev preflight and merge safety: sync-base
+
+`ccam lanes sync-base` is the ONE sanctioned merge in the pipeline — `origin/development` into a feature branch — used at Stages 2, 8, and 12. Three modes:
+
+```bash
+ccam lanes sync-base --check feat/<slug>      # read-only preflight: fetch + collision check + DEV_DELTA/DEV_OVERLAP
+ccam lanes sync-base feat/<slug>               # merge origin/development into the feature branch
+ccam lanes sync-base --continue feat/<slug>    # finish after a manually resolved conflict
+```
+
+Exit codes: `0` clean, `4` merge conflict (left in place — resolve, commit, then `--continue`), `5` migration-number collision (nothing merged — rename the printed file, re-run).
+
+Two profile declarations control it, both empty (off) by default:
+
+```bash
+# .ccam/profile/profile.env
+MIGRATIONS_DIR="db/migrations"
+GENERATED_MERGE_PATHS="api/openapi.json api/client.ts"
+```
+
+`MIGRATIONS_DIR` enables the collision preflight against a numbered-migrations directory. `GENERATED_MERGE_PATHS` gives the listed files a keep-ours merge driver (never hand-merged) and folds the profile's `regen` hook output into the sync commit — the single most common cross-lane conflict, for a repo that generates an API contract/client.
+
 ### Pipeline template: ship-feature (16 node stages)
 
 The skill uses the `ship-feature` pipeline template, which defines the following 16 stages (node IDs):