feat(roles): M18 seat-role progressive capability restriction (#45)

Role contracts (roles/<role>.json): roleVersion, name bound to filename,
tools ceiling (subset of pi built-ins), network declared (none|api-only|
open; enforced when network policy lands). Strict schema, fail closed -
a non-role document refuses resolution.

mosaic-task.mjs resolve-role: config-free contract validation, emits
MOSAIC_ROLE_TOOLS / MOSAIC_ROLE_NETWORK.

agent.sh: a declared role binds to its contract. Missing/invalid contract
refuses the launch (exit 2, names the role - the under-equipped-seat
failure mode, mirroring M17 skills). Effective tools = ceiling ∩ requested
(CLI --tools or agent.json caps); no request -> ceiling stands; narrowing
and tool-free outcomes loud on stderr. Adapters unchanged; headless M9
chain (mission ∩ task) untouched.

Ships roles/researcher.json (existing seat declares the role; without the
contract the fail-closed gate would refuse its launch).

Task suite 74 -> 88: contract resolution, wrong-kind/name/network/
duplicate/unsupported/missing refusals, ceiling narrowing E2E (mock
adapter), tool-free E2E, missing-contract refusal. Test-authoring
correction recorded in BUILD-LOG (a check that registered on one path
only, caught by count arithmetic).

Suites 24/88/14/17 + verify green.

AGENTS.md

@@ -68,7 +68,7 @@ green at every step. Not production software — a proven foundation.
 `scripts/bootstrap.sh` (idempotent) · `build.sh` · `hello.sh` ·
 `verify.sh` · `run-task.sh run <task.json>` · `release.sh
 package|activate|rollback|status` · `reset.sh` (**danger**: wipes the data
-root; triple-safety-checked) · `mosaic-task.mjs validate|run|show|list|retry|prune` ·
+root; triple-safety-checked) · `mosaic-task.mjs validate|run|show|list|retry|prune|resolve-role` ·
 `agent.sh <name>` (interactive TUI agent) ·
 suites: `test-config.sh`, `test-task.sh`, `test-release.sh`,
 `test-conductor.sh`.