feat: add local dependency graph workflows

[andrei-hasna]

Summary

• repair local task_dependencies storage through startup schema safety
• expose transitive dependency graphs through CLI deps --graph and MCP get_task_dependencies
• document local dependency/blocker workflows and add regression coverage

Verification

• Blacksmith attempted first but local CLI is unauthenticated: blacksmith testbox list -> not authenticated
• bun test src/db/tasks.test.ts src/cli/cli.test.ts src/mcp/mcp.test.ts
• bun run typecheck
• bun test src/local-first.test.ts
• systemd-run --user --scope -p MemoryHigh=12G -p MemoryMax=16G -p MemorySwapMax=0 -p TasksMax=4096 bun test
• bun run build
• git diff --check
• changed-file secret scan clean

Note: origin/main does not currently define test:no-cloud or verify:release scripts, so those gates were unavailable on this branch.