fix(#792): guard fleet roster schema errors

docs/scratchpads/ms-792-fleet-enoent-installer.md

@@ -20,6 +20,7 @@ Make expected missing or malformed fleet roster configuration fail with an actio
 - 2026-07-16: Replaced installer heading `echo` with format-safe `printf`; added a regression check for the scoped package heading.
 - 2026-07-16: Rebuilt CLI and manually verified `fleet ps` with no roster prints the initialization hint, exits 1, and has no stack trace.
 - 2026-07-17: Rebased #818 onto `origin/main` at `9ddc6fbd` (#791 PR3). The added `fleet regen` command had a canonical roster read in its sibling module; it now uses the same missing-roster guard and Commander exit path. Internal NORTH_STAR, preset, and post-write invariant reads remain intentionally unguarded.
+- 2026-07-17: RoR found that semantically invalid v1 documents still escaped as plain `Error` values. `normalizeFleetRosterV1` now preserves each validation message while converting it to `FleetRosterConfigurationError`, so its command callers use the actionable nonzero Commander path.
 
 ## Verification
 
@@ -29,6 +30,7 @@ Make expected missing or malformed fleet roster configuration fail with an actio
 - `pnpm format:check` — PASS
 - `pnpm --filter @mosaicstack/mosaic exec vitest run src/commands/fleet.spec.ts src/commands/install-heading.spec.ts` — PASS (209 tests)
 - `pnpm --filter @mosaicstack/mosaic exec vitest run src/commands/fleet-regen-command.spec.ts` — PASS (27 tests, including missing canonical roster)
+- `pnpm --filter @mosaicstack/mosaic exec vitest run src/commands/fleet.spec.ts -t "semantically invalid v1 roster"` — RED then PASS; verifies duplicate agent names are reported as `fleet.roster` exit 1 without a stack trace.
 - Instrumented Vitest coverage is unavailable because `@vitest/coverage-v8` is not declared in this repository. Each branch added in the roster guard has direct unit coverage.
 
 ## Risks / blockers