feat(fleet): seed role registry markdown library

Add one markdown role-contract per fleet roster class, modeled on the
existing enhancer.md (title / mandate / boundaries structure):

- board (front): owns NORTH_STAR.yaml; ratifies/vetoes goals; never codes/merges
- planner (front): alias of the orchestrator class; emits phased FR + depends_on DAG
- decomposition (front): splits FRs into one-PR cards via native `mosaic fleet backlog`
- code (exec): implements one card to green CI; opens PR via pr-create.sh
- review (exec): correctness/scope/coverage; approves or requests changes
- security-review (exec): secret/auth/forbidden-path second line (guard lives in pr-merge.sh)
- site-tester (exec): runtime/behavioral verification vs acceptance criteria
- documentation (exec): prose + NORTH_STAR projections; single-writer per TASKS file
- merge-gate (gate): sole approver/merger via pr-merge.sh + pr-ci-wait.sh only
- rebase (exec): owns stale / mergeable==false PRs; rebase+rerun or escalate
- operator (meta): consumes/re-raises escalations; owns the PAUSE switch
- session-review (meta): post-task retros into structured signals for the enhancer

Every file states non-merge / non-code boundaries; merge-gate names the
wrapped scripts as the only merge path. No Hermes references. install.sh
gains a confirming comment: fleet/roles/*.md seed automatically via the
existing normal sync, so no per-file PRESERVE/entry is required.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/mosaic/framework/install.sh

@@ -25,7 +25,9 @@ INSTALL_MODE="${MOSAIC_INSTALL_MODE:-prompt}"
 # User-created content in these paths survives rsync --delete.
 #
 # fleet/* — the framework SEEDS only fleet/examples, fleet/roles, and
-# fleet/roster.schema.json (synced normally). The user's own fleet files MUST
+# fleet/roster.schema.json (synced normally — every fleet/roles/*.md role contract
+# lands automatically via this sync, so no per-file entry is needed). The user's
+# own fleet files MUST
 # survive `mosaic update` (which runs this sync automatically): the active
 # roster (`fleet/roster.yaml` + any other `fleet/*.yaml`), per-agent env
 # (`fleet/agents/`), and heartbeat run dir (`fleet/run/`). Without these, an