feat(fleet): add shared role semantics

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

docs/fleet/how-to/customize-roles.md

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+# Customize Fleet Roles
+
+Mosaic resolves persona contracts through two layers:
+
+1. `fleet/roles/<canonical-class>.md` — seeded baseline contract.
+2. `fleet/roles.local/<canonical-class>.md` — operator override or custom role; this layer wins.
+
+The same shared resolver is used by profile validation, provisioning, roster-v2 semantic validation,
+and launch-time persona injection.
+
+## Override a baseline role
+
+Create a readable Markdown contract under `roles.local` with the canonical filename and class marker:
+
+```markdown
+# Code — local role definition
+
+The local code role (`class: code`) follows the operator's repository conventions.
+```
+
+Save it as `fleet/roles.local/code.md`. Do not edit generated or seeded baseline assets when the goal
+is a durable local customization.
+
+Legacy aliases canonicalize before lookup. Therefore `roles.local/implementer.md` does not override
+`code`; use `roles.local/code.md`. See [Legacy Fleet Class Aliases](../migration/legacy-class-aliases.md).
+
+## Add a custom class
+
+A custom class remains supported when a readable contract exists for the exact identifier:
+
+```markdown
+# Release notes — local role definition
+
+The release-notes role (`class: release-notes`) prepares operator-reviewed release copy.
+```
+
+Save it as `fleet/roles.local/release-notes.md`, then reference `class: release-notes` and a matching
+`tool_policy: release-notes` in roster v2. Adding only a `LIBRARY.md` row is insufficient.
+
+Names such as `worker`, `analyst`, and `canary` are not built-in aliases; they need genuine custom
+contracts. `agents[].alias`, Tess, and Ultron are display names and cannot select a class.
+
+## Validation and authority boundaries
+
+Semantic validation reads the winning contract and rejects missing, unreadable, or empty files.
+Protected authority is derived from canonical class metadata in code, never from role prose. A custom
+contract cannot claim merge, validation-certificate, orchestration, lease, or interaction authority.
+
+Roster v2 also fails closed when a protected class and tool policy do not match after canonicalization,
+or when an unprotected class claims a protected tool policy. The legacy `operator-interaction` policy
+canonicalizes to `interaction`.
+
+Role customization does not issue leases, store validation certificates, mutate credentials, or
+change lifecycle state.